Normal temp for AMD Athlon X2 2400 at 2.8 GhZ?

Those temps are fine, nothing to worry about. Each CPU has its own threshold of temperature, try googling it or looking in your manual to see what it is, but 40-50'sC shouldn't be near the max. If you really want to stress test, you can try downloading prime95 to see how well the stock cooler does. Otherwise, you can spend ~$20-30 on a cheap air cooler to reassure yourself.