It varies quite a lot on the load you put on your PC. Your bios/temperature monitoring software will probably have a setting to shut down the PC if it overheats to save damaging the CPU (not always enabled by default so have a look) If you want to find your max temperature try running a fairly recent game with settings maxed.
Do the ebuyer people say if they used the stock heatsink/fan? My 2800XP idles at 35 ºC but only because I used a zalman heatsink and 80mm fan. I would expect greater temps if I used the stock cooler.
45 - 50ºC sounds reasonable though, but I have heard that the 64 bit cpu's run quite cool but I don't know what the average is.
Do you have sufficient case fans? I've found if mine are set too slow the cpu heats up a fair bit.