At least you have two possible brand/models to hunt for! Their PCIe equivalents are cheaper, but that's the price to pay for keeping all that other old-but-good stuff!
Just keep it relatively cheap... a new mobo, i5 3470 processor, and 8GB DDR3 would still be very affordable, far faster than your 8 cores of old Xeons, and even the free Intel HD4000 video would run circles around what you already have... even coming close to the Radeon 4650 we talked about.
Sometimes it just doesn't pay to keep old stuff going.
Instead, keep your eyes open on used buy/sell forums where you could snag any number of AGP cards REALLY cheap!!