Your main limitation is that you want to keep your current video card. It's AGP, and as Duke said, choices will be limited. I agree that socket 939 is your best bet, and just stick to DDR.
The alternative is to go all out, and upgrade CPU/MOBO/RAM/Video, but then we're talking about a totally different budget.
For your £120 I would look for something like this:AMD Athlon 64 3700+ Socket939, San DiegoCore, 2.2GHz, 1MB Cache, Retail
£64.26 Inc VATAsrock 939Dual-SATA2 (Socket 939 & Upgradable to AM2) AGP & PCI-Express Motherboard (MB-002-AK) £46.99 Including VAT at
17.5% (bottom one - the others do NOT have an AGP slot; the yellow slot is for an AM2 daugher board!!)
If you want dual core, replace the CPU with this one:AMD Athlon 64 X2 3800+ Socket 939, ManchesterCore, 2x 2.0GHz, 1MBCache, Retail
£103.98 Inc VAT
I do agree with saving for a new system, with a dual core CPU, new RAM, new motherboard, and new video. How much do you think you can scrape together at the most?