go to pricewatch and search for barebones systems
I don't know how much you know about what you need but you should get at least a 2 gig processor, 1gig of RAM, and a decent video card (lots of good models on the market). I would go PCI (express) for the motherboard instead of AGP. the market seems to be moving in the direction of PCI for whatever reason.
always try to buy a better processor than you initially think you should--even another 0.2 gigs can add another 6mo or 1yr of life to your machine. for $20 or $40 more there's no way to justify not doing it--possibly if the next level up is 80 or 90.
before you buy the barebones, go back to pricewatch and search the individual components you're buying--especially the video card and the hard drives. you can install those yourself and might be able to get them much cheaper (I saved $80 doing that). I WOULD buy RAM from the barebones people even though it will be more expensive just because I've had compatibility issues with RAM several times in the past and they will guarantee compatibility.
also, try not to get onboard video on the motherboard--it has been known to cause problems down the line. you're buying a good videocard anyway!