Unless you are on a very limited budget with regards to a video card, then it isn't worth getting a card to run that way. Pretty much anything from the HD 4670 and up will be faster than any of the cards that are compatible with Hybrid CrossfireX (which unless things have changes much with the 8xx series chipset, is limited to HD 2400 and HD 3400 series cards), ATI CrossFireXâ„˘ | GAME.AMD.COM
Edit: Just looked up one of the new Gigabyte 880G boards, GA-880GMA-UD2H (rev. 2.1) - GIGABYTE
and it looks like with the 880G it will also work with the 5450 in addition to the 2400/3400 cards.
The one nice thing about using an ATI card with the AMD boards that do support Hybrid Crossfire is that when used with a higher end card makes using up to four monitors a breeze; enable both the onboard video and the Surround View option in the bios, and you can run up to 2 monitors on both the card and the onboard (which works very well by the way).