Kingston Value Ram 512MB DDR2 533MHz/PC2-4200 CL4 Dimm 240 pin Unbuffered - £25.32 or less delivered @ Ebuyer.com
Looks like a good price. Go via Quidco for 1% cashback.
Description : ValueRAM is Kingston's value-priced line of industry-standard, generic memory intended for customers who have white box or generic computer systems, or who plan to purchase memory by specification. ValueRAM is designed to industry specifications tested, is 100% tested and is available at competitively low prices.