Building a Low-end system is really not worth the time, you will probablly wind up spending more! Why not the Dell Entry system? I think its about $350, (with free shipping online) and although it has onboard graphics and only 256mb of memory, you could easily add another 256mb for $27. As for the On board graphics, they will run the SIMS, and im pretty sure the entry level Dell also has an AGP 8x port.
This will save you time, cause it will be pre-loaded and configured with winxp, not to mention any hardware woes you may have.
If you were doing a gaming rig, then of course you should build it because of the money you will save and the sheer expandability. But if were talkin a pc thats low end or mainstream, Dell will probablly wind up saving you both time and money.
If you do want to build though, you could Def get a tower built on newegg for around $275-300.