First UK computer part site I found was:
http://www.ktscomputers.co.uk/
I have no idea if they're any good or not, but for £112.31 including VAT[?] you could get a 1.6Ghz Duron (1606) w/ heatsink (1147), 256MB of PC2100 RAM (934), and a nForce2 motherboard (1567) to plug them into. Since it's only a Duron you might be able to get away with your old PSU if you don't have too much running off it... That should give you a nicer boost than your recent video card upgrade. K6-IIs were nice in the days of the Voodoo2, but you've got to upgrade the bottleneck if you want any more speed (which of course you know, but I
had to say something).
Also, I find it hard to understand how the FX could have made things so much better than the Geforce4... What exactly changed???