Should a 2.0 card work on 1.0 PCI Express slot?

PCI-E 2.0 devices are fully backwards compatible, so you should have no problems. The hardware will only function at PCI-E 1.0 speeds, though.

They probably recommend you don't use them in first generation slots for performance reasons, but the GTX550 shouldn't come anywhere near the maximum speeds of 1.x. I have seen both 8800GT's and 9800GTX's working fine in PCI-E 1.0 slots.

If you know someone that has one and is willing to experiment, you may simply want to try it before you buy it. Today I put a MSI N275GTX PCI Expres 2.0x16 video card in the 1st Gen PCI Express x16 slot of an ASUS P5AD2-E Premium. The problem you may run into is not having sufficient PCI Express Power for the video card. The power supply I have has 1-6 Pin and 1-8 Pin PCI Express power cable. The GTX275 has 2 6-pin connectors. It was not detected until both power cables were connected.

How do you fit 8 pins into 6 holes you ask? On the 8 pin connector the casings for 2 of the pins have a plastic bridge for lack of a better term connecting them ensuring it is put in correctly. You can carefully hack saw through the plastic bridge ensuring you do not go past the depth of the pin socket and then connect the 6 pins to the left of the former bridge to the normal slot.

End result is Triple performance in WOW for my daughter. Formerly 5-15 fps on Good settings. Now 30-45 fps on Good settings and about 15 fps on Ultimate settings.

The CPU for the system is a 3.2Ghz P-4. It is NOT overclocked.