Overclock.
Ram Upgrade.
Water Cooling
Larger Disk
Defragment
Delete Crap
Waste $100 for the Geeksquad to tell you what I just did and suggest the overclock setup to someone who costs even more.
The only way i'd see Water Cooling helping is if your PC is burning and it's trying to lower it's heat by lowering performance, eitherway in the UK its overpriced to get a Water-Cooling GTX 580 or anything, you can buy a GTX 590 for the price.
Plus £200 wasn't my thing, so i went with air until the future.
Curios, $600 is a good budget, if you can sell off the laptop for some more, you can make a extremely good PC, i've been using a medium-end PC for a while, i need to upgrade my GPU from a GT 430, but eh, i got it for free in perfect condition.
Overclocking is a bad idea with a bog standard cooler, plus if you are starting off you're going to forget something up and void your warranty.
Eitherway Ethan is right.
Bulldozer's around the corner and the price is going to max out about the same as a i7 2600k, so you should wait for that before building, If you're alright with waiting longer, Ivy Bridge is a good choice, if you want to finish up now, just get a AM3+ socket quad/six core, you can make good M-ATX CPU + MoBo setups for less than $400
Add a M-ATX case and you can make a proper nice LAN gaming kit if you want to goto LAN conventions, NVidia are doing some good ones this year.