A good CPU cooler (I have it myself) is the Zalman 7000A AlCu. It's for P4's and Athlons. I dunno if it's the best around, but it'll certainly outperform the stock cooler (and it's silent too!). Just make sure you take a look at the list of supported motherboards, and take a good hard look at the 7000A all-copper option - it might cool better, but is waaaaay heavier. I run a P4 myself, and have idle temperatures (in a very low-airflow case, silent-modded) of some 35 Celsius; with the stock cooler, I had 38 C. But the real difference is in the temperatures at full load, of course.
Note that your XP processor will go up to some 80 C before overheating. If I recall correctly, the XP will then slow down to stop it from burning up.
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Should you sell your videocard... hmm. Performance-wise, it should be comparable (slightly slower) to an Ati 9800 pro. I have one of those, and I have yet to be disappointed by it. What exactly is disappointing about your 5900xt? It *should* be okay for most games. What games are you running, what resolutions are you trying to run, and what are the results?