Where are you located? That card has free shipping so I guess you must not be in the 48 states. If you want a significant speed increase you will have to go with a 58xx series card or faster so the PCI Express issue is going to be there with any upgrade. I don't think the 57xx/67xx/77xx cards will be much faster than your 4890. I don't know if PCI 2.1 is backward compatible. You might try posting that question on the video card section. I have an ASUS 790X board and the 5850 works.
I'm in New York, so I need to pay sales tax on Newegg, plus the free shipping is UPS and I wouldn't trust UPS to ship a rock. I'd have to pay to FedEx it, which is always expensive...
AMD Phenom 2 X4 : 955 BE
Gigabyte GA-MA790XT-UD4P
4GB DDR3 Corsair 1333
XFX Radeon 4890 1GB
320GB Western Digital RE3
1 TB Western Digital Black
750W Corsair PSU
Thermaltake Armor+ MX Case
NEC DVD-RW 3520
Razer Copperhead Mouse
Windows 7 Pro