Don't spend too much over £50 or you might as well save up (I would anyway) and get a dual core X2 3800+ (£80+) or E6300 (£100+).
AM2 and LGA775 both (in theory) have a continued upgrade path, LGA775 will have quad core and 1333 FSB chips, AM2 65nm is both already out and still on way (though performance atm is nothing to write home about) but will hopefully have some new models in the future (despite AM2+/AM3/4x4 and who knows what other new sockets they are cooking up at AMD
).
In keeping with your budget plan any of the nForce 550 or nForce 4 boards should be fine for stock (and are around £50), if you intend to overclock I don't have a clue about which AM2 boards are decent.
http://www.overclockers.co.uk/productlist.php?groupid=701&catid=5&subid=805
Memory wise for stock PC2-5300 (667 DDR) covers all the bases for either an LGA775 Core 2 Duo (and future 1333 FSB Core 2 CPUs) or AM2 Athlon 64/X2 rig and can be had as cheap as any of the DDR2 memories (don't bother with PC2-4200, the price is no different really). For overclocking I can't help you with AM2, for Core 2 Duo you'll want PC2-6400 (or higher depending on the overclock your going for and the CPU multiplier/max motherboard FSB).
Corsair value is pretty reliable and has a lifetime warranty, latencies (or clock speed for that matter) don't make a big difference to performance and especially if your staying at stock there is no need to consider anything more expensive than about £150 for 2GB. I'd strongly suggest you get 2GB now though (if you are a gamer), budget or not, because compatibility is a big issue with Athlon 64/X2 rigs and they just aren't happy with mismatched memory (that includes what is apparently the same model and that isn't in a dual channel pair) and you *will* need it sooner or later (if not now, many games have already made 2GB pretty much a requirement for smooth gameplay at high settings).