At least 1Gb, but I would go for 2Gb, big hard drive - 160GB or bigger, at least a dual core CPU, Nvidia 8800 based graphics card (or 2 in SLI configuration) if you can afford it.
And a quality motherboard and powerful PSU.
You are looking to pay between �100 and �200 for a quality motherboard, but if you want top gaming performance then this is what you need.
You can also spend between �50 and �100 for a good PSU.
The trouble is that if you want good performance for current games, and a PC that will cope with upcoming games, then you have to pay for it, there is no real cheap option, obviously you would shop around for the best prices.