Your system overall is pretty decent, so that's good news!
If you're looking to spend the least amount of money while you can still enjoy SH4, I'd recommend you consider these:
NVIDIA 7600 GS (least expensive, wouldn't consider anything below, but this is really stretching things)
NVIDIA 7600 GT or Radeon X1650 XT (for a few dollars more, slightly better performance but still not ideal)
If you can afford it, I'd suggest you consider a Radeon X1950 Pro or an NVIDIA 7900 GS. I believe either of these two cards should give you the best cost/performance ratio for running SH4 reasonably well.
Food for thought.
