and, as the thread suggests, was wondering what would be the 'best' the board can handle. As it's the original PCI-E and not the newer revision, something like an Nvidia 480GTX would probably be a bit of a waste, though I am planning on upgrading the CPU and RAM as well, so no worries about bottlenecking. Cheers!
No worries, even the most powerful current GPUs will have plenty of bandwidth on a PCIe 1.x 16x slot.
Currently the 'best' GPU you can get is the GTX 580, although you would probably be better served with a much less expensive GTX 570 or 6970.
Just be sure to check that your PSU has enough amperage before dropping a monster card into your rig!