Unfortunately replacing the ATI 3200 integrated card won't be possible without replacing the entire motherboard. Check out this quote from a HP TX2500 review:
"... since the Radeon HD 3200 is an integrated graphics solution, it suffers from the same potential problems as any IGP. Namely, it shares the system resources (RAM) and is soldered directly to the motherboard (so it cannot be replaced without replacing the entire motherboard)."
However, make sure that your graphics card is indeed performing as fast as it should, since the editors in that same review had troubles achieving 3DMark05 scores that others were getting with the same ATI 3200 card:
"After a week of constant attempts by our editorial staff of technical experts to diagnose the problem (including multiple driver updates and replacing the system RAM), it was determined that the Radeon HD 3200 in our review unit was not operating properly due to a hardware malfunction. In other words, our tx2500 is going to have to go back to HP for repair or replacement."
I'd suggest downloading 3DMark05 and running the test. You can also try allocating more of the RAM to video performance by changing the settings in your BIOS. Just be careful when messing around in the BIOS -- when in doubt, exit without saving! :)