Remind me not to get an AMD Athlon series processor - they have too many variants that use the same model number. From what I can find, it looks like the 531s uses socket AM2, but I'm not even entirely sure of that (there are a couple of X2 3800+es that use socket 939). There are three variants of the X2 3800+ that use socket AM2 - one has TDP of 89 W, one is 65 W, and one is 35 W. I can think of some choices for you now:
1. Play it safe and figure you've got the 89 W model.
2. Remove the heatsink and try to read the part number from the cpu (e.g. ADA3800IAA5CU for the 89W model according to Wikipedia)
3. Call Dell and ask them.