My computer is this:
AMD Athlon X2 2.6ghz CPU
2gb DDR2 (Kingston I think) RAM
80gb Hard Drive (that needs upgrading)
nVidia GeForce 8400 GS 256mb Video
It would cost about £100 to build that or maybe even less, it runs Blockland fairly well. The specs of this one you posted on craigslist are fairly stuffe.
If you want a decent computer I'd reccomend you build one, if you live in the UK (which I doubt) a good source of parts is a firm called MicroDirect which is based in Manchester (they deliver), in the US I heard the place to go is NewEgg. Not only is it cheaper to build one you know what's in it so you wont be dissapointed.