for the UK, here are some links:
These brands seem to be available locally:
Costs more than a newer Mac Pro but it does support 8x4GB.
Amazon USA has 2x2GB $40 though not on UK site, best quality (what I have) and price.
Mac Pro 1,1 thru 3,1 667MHz FBDIMM with the large heat sinks and top tier.
And if you really do use CS etc then 24GB would be desireable.
At the least, add 4x2GB, though 8x2GB would be even better.
A good fast disk drive for scratch never hurts, and WD 10K VR has new 180MB/sec line in 250GB US$100, slightly more like $130 for 500GB model. Great for system drive (the 250GB, perfect size).