I'm not saying its completely neccessary. I'm saying that for 60 dollars, that's really a good deal on something that outperforms most, if not all of single core processors. Most people will gladly toss in an extra 20 bucks, and reguardless of the use of the computer the processor will usually be the most valuable part.

Now if you are a gamer, here is a good suggestion of graphics cards:
Heavy gamer: Nvidia Geforce 8800 ($280-600 depending on memory)
Moderate gamer: Nvidia Geforce 8600 GT (Will play anything and everything for the next 1-2 years, even games like Crysis on at least medium settings for $100)
Minor gamer: Just buy a motherboard with an integrated graphics card, won't cost any extra

You probably will need 2 gigs of ram if you are an avid gamer, 1 gig if you are not, reguardless of whether you use vista or not. If you are using xp or linux you can skimp by with half a gig.

Motherboards just depend on what parts you want to stick in the computer. They can range from 30$ to 200$.

All in all, you will spend anywhere from 250$ to 2000$ just depending on how powerful you want it to be. You can have a good, moderate gaming computer which will play all modern games and games for 1 year from this date for a good 400-500$.
I would not advice going single core unless you want your system to be gimped when it comes to gaming, although it won't be a huge problem unless you play games like crysis. But I'm willing to bet you want to chuck in an extra 20 bucks for it.