I would recommend one of the Asrock Upgrade mobo's if you are set on keeping the XP but keep your options open as the Asrock board has a connector that allows you to stick a K8 A64 onto a riser card and hey presto, you are in a couple of minutes running a 64 bit system.
I priced it out though on eBay and did so for less, in that for a £100, you should be able to have a decent 64 bit system brand new, Tekheads were recently knocking out AM2 Semprons for £28 whilst I just bought a 2800+ Sempron 754 from Komplett for £35 (£41 delivered), somebody sells brand new MSI K8T Neo FISR's on a certain auctiony type site for £20 and a stick of PC3200 RAM can be had quite cheaply as I bought a GB stick of Rendition PC3200 for £30 delivered on this "auctiony" site. One word of warning though with 64 bit Semprons is to avoid the 128k L2 versions like the plague, I have just crushed a core on a Clawhammer 3200+ with a 1mb L2 and got that from this same place for £30 but because the 3200+ had 1mb of L2 cache it went like the clappers and was time and again being ranked in real speed benchmarkig above FX's and dual cores.
Alternatively, if you are set on sticking with Socket A, this "auctiony" site I mention not, does EPoX 8rGA's for about £20 delivered brand new and the one I had was a brilliant board knocking AN7's to pieces and is a 100% overclocking dream, you will have to change the Northbridge heatsink on these EPoX's but once you do (the MSI heatsink would fit straight on) despite it being a IGP based nForce2 board, if you use an AGP, there is no reason why you couldn't overclock a T Bred to 2.5Ghz without it even getting hot. The 8rGA seems to be a bit of a overclocking odd bird in that it has the A3 revision MCP-Turbo Nforce2 chipset whilst the 8rDA's have the A2's as does some of the 8rGA+ which being a newer board you would think was a better board but the plain 8rGA performs better than either sister boards.
If your CPU is T-Bred then the 8rGA will be a good choice as more likely or not it will overclock your chip without you doing a thing, my 8rGA detected my 1700+ AUIGA stepped Athlon XP as a 2000+, a 300Mhz increase and 100% stable...
AO