Unfortunately, I do not have much good
(read 'cheap') upgrade news for you...
The HP 3260 ships with a Pentium-200MMX CPU, which is directly
upgradeable at most to the 233 version using your current motherboard.
This is the limit of your CPU upgrade options, due to the 66MHz bus speed
limit of the motherboard, unless you wish to consider
a third-party CPU upgrade.
Your motherboard upgrade options are even more limited. You will not
likely be able to swap in a standard size (ATX or even Baby AT form
factor) motherboard to get one with a higher bus speed capability, let
alone an AGP slot. Still, check with your local HP dealer for advice
regarding just what size of motherboard will fit in your case. I have my
doubts there will be many options, though.
Happy computer hunting... ;)
Tip: For more advice on what you should consider in the current
crop of CPU's, keep visiting The CPU Scorecard.