Performance Bottleneck?

Hi,

First off, I've read the various articles and guides on system performance and followed their advice. However, I still have considerably below what I would expect to get on a 3DMark2004 benchmark test. I'll describe my system first, then I'll let you know my score. I think it's much lower than it should be..

System:
Retail Athlon 64 3200+ (2.1GHz) (which runs at about 47DegC on average)
756Mb DDR RAM (1xCorsair 256MB PC2700, 1xUnbranded 512MB PC2100 - presumably brining the Corsair down to PC2100 too)
Excalibur (HiS) Radeon 9800 Pro 128MB.
Asus K8N-E Deluxe mobo, utilising its onboard sound.
A bespoke network card connected to 76KB/sec broadband connection through a 100BaseT Ethernet RJ45.
2xHDDS - one housing Windows which is only 5GB and a West.Dig. 5200RPM, the other a Maxtor 7200RPM 120GB drive which my apps run from.
Software:
Windows XP (32bit version I'm afraid..)
ZoneAlarm is always on, but when playing games, it has "Stop All Internet Activity" enabled. I've tried benchmarking witout it, and my modem unplugged, and the difference is not significant.

Now, my 3DMark2004 score is 5225. Of the benchmarks I've seen, it should be running at at least twice that. This is significant enough for me to think that no amount of S/W tweaking is going to sort the problem out. I'm not sure where the bottleneck lies - maybe its the RAM? - but I'd very much appreciate any ideas.

Thanks