If you want to run on high settings you are going to want a computer that has a graphics card. The CPU speed isn't going to matter as much as the graphics card.
The game's recommended settings are for a Geforce 8800 gt card, they run under 200 bucks.
You could also go with a slightly better gtx 260 if you wanted a geforce GTX model card.
You can keep your eye out for a computer with a good graphics card, but usually those pre-built machines will have a lot of good stuff in them and will run you near a thousand bucks.
likely you will buy a cheaper computer and add the card in after.
it'll be a hard budget to work with for needing a graphic card that takes 100 watts… You'll need a good power supply and a decent motherboard and cpu.
performance costs money. Best of luck