take it out. look at it. tell us just what graphic card is in there you are trying to use.
that should always be you know like step #1
there was no "late 2008" just Early 2008 or better yet "3,1" or similar designation.
It came with OEM 2600XT and those can be flakey. Could be they used something else. 8800GTs die too.
The Apple ATI 5770 is the best safest bet $249, requires 10.6.5 but there is NO such DVD, only 10.6.3 sold retail sadly.
FBDIMMs have gotten cheaper by 10:1 and worth looking at a set just to be sure.
Its a GEForce 7600 gf. As regards the build date I got it by putting the serial number into the search box here - http://www.appleserialnumberinfo.com/Desktop/index.php It told me it was built in Cork in Oct 2008. Not that it really matters. I am simply canvassing for opinion. I don't expect anyone to say 'yeah. that will work'. I could always take it to the Genius Bar. It only cost me £150 with a functioning 20" cinema monitor so I have something to play with financially.
There was someone else not that long ago with one iwth 7600.
Apple never sold or supported a 7600GT
Older Mac Pro use a different EFI and had a 7300GT, that was the OEM for 2006-7.
A GT120 if you can find one, Apple OEM for the 2009s, is about $120US and ebay USA West Coast MacVidCards is a good source... but you are in UK.