Have a look at one of the sites where you can buy the individual parts, make up a shopping list of the same parts (or equivalent) and see what the price comes out to. This will give you a rough idea.
The only things worth looking at to have a nice gaming unit would be to get two SATA drives to run in RAID 0 as your C drive, then use the 500gb for storage. I've got 2 10,000rpm Western Digital 74gb drives which are pretty fast. this would be pushing the price up a bit though.
Also, look at maybe getting a gaming mouse (I use the Logitech G5 which is rather good), although this depends on what sort of games you're playing. It's great for FPS games.