2008 iMac 24" Graphics Card

You can only swap Intell iMac video cards with iMacs of the same model. In your case, only other aluminum 20" and 24" iMacs. Unless that card came in an iMac of that model, it won't work.

I suggest staying away from the ATI cards. Ther are lots of people including myself having problems with 10.6.7 and newer freezing with the ATI cards.

no problems here

No problems with my 24" iMac ATI 2600 Pro with Lion or Mountain Lion runs flawlessly.

Double check that video card

I think that 7300 is for the 2006 imac, likely not compatible.

I don't want to necessarily encourage you to do this but i've had good luck reflowing those 8800 video cards in the oven. There is a video on how to do it on youtube and so far I'm 3for3 in success (still working a year later).
Not for the faint of heart and does require some specialized tools and sone skills.

Alternative is Blue Raven tech in Philly. You can send your card in for reflow, less than $200 if I recall. Might take two weeks.

Good luck.

Jb