[Build Help] Trying to upgrade old graphic card. Somewhat clueless.

I posted this in r/computers but didn't get much help, so I'll try here.

I have a shitty EVGA 7300GT 512MB in my comp right now (yeah, brutal). I'm looking to upgrade it to EVGA 520 GT 1024 (yeah, not much of an upgrade, but I'm broke). How can I tell if I can easily switch the two cards? I've opened my desktop, but I just can't really tell if it's possible. What do I need to look at specifically to figure this out. Or, is there a better graphics card than the 520 for around the same price? I'm only looking to spend around $70ish.

It was suggested I check out Radeon HD 4670 or a GeForce 9600 GSO, but I don't have low profile and the 9600 is another 512MB. My shitty card right now has these dimensions: 11 x 2 x 8 inches.

Thanks for any help.