Kingston 1GB DDR2 533MHz/PC2-4200 Non-ECC CL4 1.8V Unbuffered Dimm 240 pin - Ebuyer QF 64183 - £30.04 (£22.62 via Google-checkout with super-saver delivery)
Ram prices keep on falling, if you have or are planning on getting Vista now is a great time to get a couple of these (make sure you do it on two seperate orders to make the Google checkout savings...)
Other good deals:
Crucial DDR2-533 1GB (QF 63618) £23.56 delivered (both Kingston and Crucial are reliable brands)
Kingston DDR2-667 1GB (QF 91121) £24.57 delivered (faster, probably best choice if your motherboard supports it). This has been posted before but price has dropped further recently.