Hey, don't call the 7600GT crummy, it served me very well for quite a few years!

Still, it is getting rather old. Depending on the resolution you use (if you're happy with 1024x768, it's less of an issue), you'll have to upgrade soon if you want to play new mainstream games, though you probably didn't need me to tell you that.

Have you considered a second-hand 9800GT/GTX as an upgrade, if money is tight? They go for really cheap these days, and they are significantly faster than the 7600GT.

I have an early PCI-E 1.0a port on my motherboard, and there are known issues with them and anything above PCI-E 1.1 (even though they should in theory be compatible with later PCI-E cards), so basically I'm stuck with this card until I buy a better motherboard, and when i do that I'll probably want to buy a better processor (do modern motherboards even support P4?).

No, they don't. A new motherboard means a new CPU and new RAM, and you'll probably need a new power supply too. You can keep HDDs (good thing too, considering their prices have skyrocketed over the past few months), optical drives, case and peripherals, though a modern monitor is a good idea too, if the one you use is more than 3 years old.