AMD's A64 TDP ratings take the opposite approach from Intel's for the P4: They cite the highest power that can possibly be produced by any processor of the particular model that comes off the production line. In other words, the highest power variant. So they almost always read high, higher than in actual stress testing on a real machine. For more details, see Athlon 64 for Quiet Power
There's some difference between a 2.0/512 and a 2.4/1G, I am sure, but if you have even a bit of headroom in your cooling system right now, you're probably not going to notice much difference. So what if temps go up a couple degrees, right?