The HCL is a ton of forum posts going back years, one per item of hardware - not the most helpful starting point
In my experience:
* most "mainstream" motherboards will work, some clever/high end features on the expensive motherboards might not. I have no idea which work "better" (but would love to know) but if you buy things in the "normal" prices (i.e. nothing super new or hard core) they just should work. Pick the ones that are on offer, go on the manufacturer's website and check if they say anything about linux compatibility (the HCL here seems quite out of date on boards)
* wired network cards will work - i have not encountered one that didn't in ages
* any normal drive will work - pick the sweet price point
* if you buy a dedicated graphics card, at this moment (sep. 2009) nvidia is a better bet. The same tends to go for built in onboard cards. Most of the time if you dont plan to play 3d games then pretty much all cards can be made to work. It can get tricky as some of the shipped driver versions are iffy it can be a pain if your distro choses to boot straight to that and freeze (see below). In all cases another version of the driver will work.
* most soundcards work, including USB ones, the tricky bit is sometimes the input (microphone) - that is something you might need to look up if you want voice chat
* if you want wifi, check the HCL as not all work - better to buy a PCI or USB wifi card that is known to work as many on-board options do not.
* if you are going to use fancy input tools (i.e. gaming keyboards, that kind of stuff), some keys might not work unless you spend a lot of fiddly time creating a keyboard definition. The normal keys will work, though
* webcams are a lottery, built in ones rarely work so again look at the hardware list and pick a USB one that is known to work
As I said earlier some of the versions of graphics drivers can be buggy, and some distributions are too helpful and go straigt to the special driver not allowing you to set (or revert) to a more generic one if the specific one doesnt work. It can get annoying so make sure you choose a distribution that has the option to boot to command line for your first attempts as it is a pain to try to fix a distro that boots to GUI then freezes. You can always install the one you really want once you have figured out the drivers that work and don't work.
It gets better with every release of the kernel