The minimum requirements for XP are a PII (300Mz) and 64 MB of RAM. But, you wouldn't like the results with that set-up. Microsoft recommends 128 MB of RAM. I've had XP up and running on 2 boxes with a AMD K6 and it was pretty good. Also installed it on a PII and it was slow, but usable.
To answer the original post, most Linux distros should work without too much of a problem. SuSe is very easy to install and use, so is Xandros. But, you also want to consider how much HD space you have. If space is limited, you might be interested in Beatrix Linux, it'll install in less than 200MB. The entire OS will fit on a mini-CD. Also, if you're REALLY space limited, you could check out DSL (Damned Small Linux), I think it runs on 50MB.