What is the bottleneck when playing games these days? Is it usually the motherboard or bus or video-card? I mean if you're playing a near cutting edge 3D game the CPU has power to spare, right? I think I'm going to buy some parts and I'm wondering which ones where it's important to get good stuff which parts I can get cheaper stuff.
EDIT:
If anybody out there plays new games like Starcraft II, etc, and knows about hardware and wants to recommend processor, video card, and motherboard, go ahead. I'm browsing newegg and feeling kind of bewildered.

The bottleneck is hard drives.

They have been for a while. Spinup 7200RPM hard drives transfer data at a rate of around 50 or 60MB/s. That's okay if you're on a 100BaseT network where the transfer is 11MB/s. But with a 1000BaseT network (gigabit) the transfer rate is 120MB/s which surpasses your "average" hard drive.

You could go with multiple drives and using RAID levels for disk striping. But IMHO solid state hard drives are the thing to have. RAID is better used for redundancy.

Right now the fastest SSD on the market is the Crucial C300 (lookup benchmarks). With a transfer rate of 355MB/s. I've opened Adobe CS3 in less than 1 second on a Lenovo X201 laptop with a C300 in it (Core i5 version).

Price has gone down on them considerably. Since I last bought one it was over $600 for a 256GB. I know people whine about the price/GB but you're paying for the speed and not capacity. If you want both then go with tiered storage. My home computer OS runs on a C300 but I have a 6TB hard drive array hooked up to it for storage.

Now my games load in seconds instead of tens of seconds.

Metro 2033 loads in less than a second.
STALKER: Shadow of Chernobyl loads in a couple of seconds.
Elder Scrolls: Oblivion loads in 3 or 4 seconds.

Need I say more? If you have a game you want me to check out then I'll tell you the load time for it. I have hundreds.

My boss at work bought two 64GB C300 SSDs and RAID 0 them. He gets 700MB/s transfer rate with them (measured with dd on Linux).

How fast does it load nethack?
EDIT:
Oh man it's going to take me forever to get my bearings on all this new stuff. I wonder if I could just get everything pre-packaged since I don't know what I'm doing and then swap out the video card.

All I ever really played in the olden days was Civ IV. These days I guess I want to give Starcraft II a spin. It's rated platinum at wineHQ.

Provided your system is relatively recent, the graphics card will usually be the bottleneck, especially if you have a store-bought machine or ordered one of the pre-configured systems manufacturers offer. They tend to be lacking in that regard.

I play Starcraft II, and it runs quite well on my system, which has an AMD Phenom 9950, ATI Radeon 4850, and 4 GB of RAM. That processor and graphics card are probably not widely available anymore (it's been around a year or so since I've updated my hardware), but you can look at benchmarks on many websites to find parts of similar (or better) performance, which will probably be less power-hungry and produce less heat. Also, my graphics card is a single-slot type, which I wouldn't recommend, given the large amount of heat that this model produces, which is exhausted into the case instead of out the back like a double-slot card.

On a bit of a tangential note, if you're building a whole new system, or will be considering your case ventilation, I'm a big proponent of following the positive pressure rule for case ventilation. Almost all decent cases will have filtered intakes on the front for the fan, but they will tend to have unfiltered holes in the back, on the sides, and sometimes in the PCI slot covers. By ensuring that your intake fans push more air into the case than your exhaust fans remove, there will be a slight positive pressure in the case relative to outside, which will push air out of the unfiltered holes I mentioned, and will only draw air in through the filtered intakes. This greatly reduces the amount of dust you will collect inside. It's awesome.

EDIT: Hard drives, too. I didn't consider that one, but sag47 is right, that is a huge bottleneck, both for games and general usage. The graphics card thing is more specific to games.

heh, nethack load time nonexistent.

The most important component for gaming is the graphics card. Of course, make sure to have a mobo that can support the graphics card well enough (PCI-E 2.0).

PCI-E x16 2.0 cards are backwards compatible with PCI-E x16 1.0. So that doesn't matter unless the OP has AGP.

I still say the biggest bottleneck is hard drives (assuming OP already has a gfx card capable of gaming).

Once the level is loaded the hard drive doesn't matter, does it? I think that would just make it load faster. After that I think the video card would be the bottle-neck. I guess I mostly want to reinforce my theory that I don't need a cutting edge processor because the video card will have a hard time keeping up with it anyway as long as I don't get a 486 DX2 or something.

It depends on what games you are running. For open-world games, like Gothic or GTA IV the harddrive is a bottleneck, because they do seamless loading of levelparts in the background. This can lead to stutterings when your harddrive is slow, but it can be partially solved by increasing your amount of RAM, going from 4 to 8GB gave me some more FPS and more "smoothiness" in GTA IV.
Most times the graphics card will be the bottleneck, but that also depends on the game, and what quality-settings and resolution you choose.
But I have also seen the CPU as bottleneck, in games like GTA IV and the newer parts of the Anno-series (Anno 1701 and Anno 1404). Going from dual-core to quad-core gave me a huge increase in performance.

So the answer is: it depends on the game.

I'm reading about RAID for the first time. If I just got one hard drive at 50MB/s could I play Starcraft II just fine? Or do I need to learn about RAID and get multiple hard drives? I'm wondering if I can skip learning about RAID or if I would be a fool to do that.

Starcraft 2 will play fine. The hard drive bottleneck most affects how long the levels take to load.

Oh good, I didn't want to bother learning how to set RAID up. Learning hurts my brain.

??? Maybe you shouldn't be using Linux if this is the case. RAID isn't that hard to setup. However, I recommend against RAID0/AID0, unless you don't have any important data.

Nothing wrong with RAID 0 if you do your backups.

Yes, backup very often. I mean if one drive dies, you lose all data.