You could use either one. The one on the right is from a AMD Athlon 64 3500 and the left is from a Opteron 185... What really matters is the series of CPU, in this case the "XP" and that in turn means the mother board needs the correct mount to hold the heat sink.
Look at your heat sink, it should look like the one on the right.
How about this... I have the AMD Athlon 64 3500 and its heat sink. They both would work in your computer. The CPU will give you 3x+/- the speed you have now. Think of a price you would like to spend on a CPU upgrade and I will send it to you (the CPU was used for 8 moths with a "slight" over clock, never a heat problem, I don't OC my stuff to damage it). I replaced the 3500 for the 185. Thank about it.
EDIT... Just to make sure, find your mother board info so we can check the CPU will not be a problem.
Also I have some work to do outside, so I wont be back in for sometime.