I bought the fastest RAM I could afford (OCZ 3200 CL2, Dual Channel) and the best Hard Drive I could buy at that time, wich were 2 Western Digital 80 GB drives, Special Series with 8MB cache (I know that we now have SATA, but it was before SATA).
Next and because the cash was a bit short (the RAM and drives were expensive), I bought a AMD XP 1800+ (at the time it was the lowest one I could find) and overclocked it with Air Cooling to a AMD XP 2600+.
Believe me, this 3 things make a big difference in Rendering Times.
I also bought an already used ATI 9700 Non PRO and overclocked it to a ATI 9800.
So, you might do the same kind of business.
I would bet on the Hard Drive and RAM.
Secondly the CPU.
And last the GFX.
You can get good already used stuff at low cost, wich will help you improve your rig.
Also, with some good cooling you can get your CPU and GFX to run faster. That’s what I did and haven’t spent a whole lot of cash on it.
Just my opinion.
Note: If you are going to use Linux ( I use it, it’s faster ) go for a NVidia Card, such as a FX 5600 or even a FX 5200, already used, they are unexpensive and would boost your feel on Blender responsiveness.