In summary, here is what to look for:
CPU: If you go Intel, try to get the Pentium D (Dual Core), or if you go AMD try to get the AMD Athalon X2
RAM: The details behind this depend on whatever motherboard you get, i.e. PC3200 for most AMD rigs these days, vs PC5300 or similar for some Intel boards. The constant that remains is you should try to get at least 1GB, or 2GB if cost allows for it.
Hard Drive: Although IDE would be ok, try to go SATA. The price difference is relatively non-existent, and it gives you the ability for more growth going forward as certain motherboard manufacturers will be phasing out IDE controllers soon enough.