Bottleneck Options

[Qrbrrbl]

Ok here's the deal. I've had my homemade system for about 3.5 years now, so obviously performance isnt exactly top notch any more (though its still a very capable system). Only problem is, I have no idea what the bottleneck component is. That goes beyond my hardware knowledge. So, heres my question. Based on the specs below, whats my bottleneck (as in, which one will give me the biggest improvement per £).

In no particular order:

CPU: AMD Athlon™ 64 X2 Dual Core Processor 6000+, MMX, 3DNow (2 CPUs)
Memory: 3072MB RAM (2x GEIL 1GB, 1x generic cheap 1GB, dont know the model or how to find out without taking them out)
OS: Microsoft Windows XP Home Edition (5.1, Build 2600)
GFX: Sparkle NVIDIA GeForce 8800 GTS 640MB (running at 1440x900)
Mobo: ASUS M2N32-SLI DELUXE
HDD: 2x Seagate Barracuda 7200RPM (one 250GB, one 500GB)

Any suggestions?