Kronozard :
I'm adding on to my old Compaq desktop, so when I checked the upgradeability it states it can only go up to E4400. You can check more info here -> http://h20564.www2.hp.com/hpsc/doc/public/display?docId=c01297188
Yeah, you're right. That's a shame.
I've been having a hunt for you, but it's not straightforward. That motherboard you have is standard mATX, so you might be able to pick up a second hand CPU+Mobo combo on the cheap and swap it in, but that would probably require a new windows license, plus you'd need to be quite specific for a DDR2 mATX mobo, unless you're prepared to get new RAM too.
Once you get to that level of complexity & cost, you may as well just pick up a whole second hand build... but that's money you might not have.
Do you have the 750ti already? Or is that what you're trying to decide on purchasing? If you've got it, try it and see how you go. It might be okay. I honestly can't say for sure.