Buy a Good Case. one thats big and you can use for years. I recomend coolermaster as a brand.
next you wana invest in a motherboard and cpu. Go for a corei5 cpu and make sure your getting a motherboard thats compadible with the cpu. this mean you need to look for a p55 mother board.
once you have these things you can look to see what you already have in your computer to harvest. like your dvd drive harddrive, powersuply. your ram probly wont be worth a damn, unless its DDR2, but really you want DDR3 ram check the mother board you buy specss to see whats compadable.
if your power supply is anything lower than 700w id buy a new one. atleast 850 - 1000. this will insure that you have enough power for everything you add to your computer. sense you want a gameing rig. most GPUs(graphics cards) that a great use alot of power. make sure powersuply is compadible. dont buy a cheap power supply! this is the heart of your computer!
last you shold buy a GPU. make sure its compadable with your mother board. get PCI ex 2.0 with DDR3 speeds with atleast 1 gig memory. this will be expensive.