63C is too high, even under load. I've seen many athlons become unstable at around 60C. I would try to get it down to 55C ABSOLUTE MAX under load. It depends on lots of things. Make sure:
1) You have the heat sink seated in the right direction. The raised corner of the base of the heatsink should coincide with the raised plastic of the socket.
2) You used a THIN layer of thermal goop. Don't put it on with a trowel. Silver compound is better than cheap white goo or phase change material, and much easier to clean off.
3) You have cleaned your heat sink and fan to get rid of dust.
4) Your fan on your heat sink is running at normal speeds
5) You have both intake (bottom) and exhaust (top) case fans.
6) Your room isn't too hot.
Sometimes you just get a mb that is hard to work with. I have an Athlon XP2100 in an Asus a7a266 motherboard. I have an all copper heatsink with a high power fan, 2 vantec tornados, one high power YS fan, an exhaust hole custom modded onto the top of the pc with a fan, an intake custom modded on the side to blow cool air into the heatsink, and premium silver thermal goop, and I can't get this computer under 56C load in a hot room or high 40's if I open all the windows. Sometimes there's just not much you can do, some mb's are more prone to heat buildup. It's at 56C now and it sounds like it's about to take off.
You should, however, be able to get well under 63C with more aggressive cooling.