I agree with others that trying to crossfire a Kabini wouldn't be worth it. I also worry about that little guy making things CPU limited. (Maybe Mantle can help that....we'll see)
I think your budget is in the right range though. I surely wouldn't spend more than $100 if it's being paired with that CPU. You can get a 7790 or 650 Ti for $100 after MIR if you catch them on sale. Keep this performance graph
in mind when buying (and that the 7770 is about 25% faster than the 7750 GDDR5). IMO, 7750's are attractive only because they don't require external power. If you look at pure performance/price, going from an $80 7750
to a $100 7790
gets you well over 2x performance.
Disclaimer: I bought a 7750 (DDR3) on newegg open box for $31 after MIR that I gave to a friend for Christmas. It plays Bulletstorm, Grid2, Tomb Raider, etc at 1080p, low-medium settings, 35-40fps (Portal 2 medium-high settings, buttery smooth), hooked up to a Core 2 Duo E6300. The price was right, but it was difficult to get things looking ok at 1080p and retain playable framerates. Lots of tinkering required. I think that E6300 was introducing some CPU limitations.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod