I don't know about writeback, but you might want to add noatime to your fstab. This will post a noticable difference in your speed (slight, but noticable is good), because there won't be a write action everytime you access a file. Writeback is alright if you're going for raw speed, but there's a chance you can also throw bad/old data into good data if the system crashes or the timing get's messed up. I'd avoid it unless you absolutely have to have it.
BTW, yes, you can do a lot of wild things to make a linux system "the fastest" on your block, but a lot of times there's a tradeoff. For instance, you can increase the data processing speed heavily just by not starting X-windows, but that limits what you can do with the system. I'm assuming you're fairly new at linux, so if I'm wrong, sorry. But if I'm not, you might want to question speed as a pure end to a means. Most of us from an MS background jumped through hoops for speed (water coolers, tweaking power, etc.), but in a year, all our money was pointless, because that old "fast" chip wasn't impressive anymore.
So, instead of worrying so much about speed, why don't you explore Linux's flexibility and power. You can do some stuff with Linux that an MS box can't even pretend to do, like clustering, being a REAL server, powerful graphics/video editing, etc. There is plenty of software to explore as well. And if speed is an issue, but you don't want to lose the power, stability, flexibility of your system, you might want to try Gentoo. It can take a LONG time to set up (think days, not hours), but then you'll have software compiled exactly to your specs, which pretty much equals speed.
Once you do some of this stuff, you might find that your desire for more speed is balanced by the realization that while speed is good, a slower, powerful train always wins a head to head battle.
