you should be looking at a minimum of 512mb for windows xp nowdays but if you can stretch the finances to get 1gig the all the better.
my advice is the 'original' pc3200 in either flavour 512mb or 1gig should do you fine. the corsair and kingston brands are favored by overclockers that wish to wring out every last drop of performance from thier systems and are top quality ram. its up to you if you wish to pay the extra.
£24.26p extra for corsair may be worth it if the 'original' ram is generic unbranded.