CPU is a Xeon E5 2670, which apparently supports PCIE3.0, which is what the GPU slots are marked as on the motherboard (That's one of the nice things about the Dell MB. all the slots are marked with type and wattage available).Looking at the BIOS, apparently it can be switched to do UEFI or Legacy. Card length shouldn't be a problem - Neither the optical or HD cages are in line with the GPU slots (don't know if there was another HD cage originally, but there's only one HD cage at the end of the PSU with room for 2 x 3.5 drive trays now).
Sounds like that could be worth looking at when supplies improve
At the moment, the card in my core 2 quad box is a GTX 550 ti, so shouldn't be too hard to improve on in the meanwhile; I don't want to swap the 550 ti over at the moment since that'd probably break a lot of music plugin license setups, which I don't want to move over to the T3600 until I've finished upgrading bits of it so that I only have the hassle of relicensing things once. Or twice if I eventually get a 3060 or similar