I'll throw a few things out there...
Get a good power supply and case!!! The longevity of your system is dependent on these two items. The system may be obsolete by the time a cheaper PS goes, but you still may have use for it or want to be able to access documents on it. I speak from multiple experiences.
Vista is inevitable... so dual core, a 3d card with DirectX 9.0c support, 2 GB RAM and maybe even 64 bit will see your computer into the future. I don't see a need for Vista just yet, but in two years the user base will be large enough that you'll basically need to switch to get your hands on the latest software and hardware upgrades.
I personally would go to a Canada Computers or Logic Computer House to buy my systems. However... if you're not completely tech inclined I recommend you go with a store-bought ready-made system for support purposes. Although the CC and LCH guys are usually nice, they don't have the best customer care policies. I recently bought a power supply from one of them and it turned out to be a bad unit. Rather than replace it as a Futureshop or Best Buy would do, I'm waiting 4-6 weeks for the warranty replacement. Ridiculous.