Opteron 1xx series CPUs are not that expensive. 2 GHz dual core Opteron 1xx can easily keep up with Pentium D 3.2 Ghz. Plus plain DDR400 is good enough for Opteron, which should cost less than DDR2 667. Opteron will also run cooler and use less power.
If you need another system few months later, then keep an eye on
Conroe