Counter Strike GO uses the Source engine. So, the performance will be similar to Portal 2 and not very demanding unless you completely max out the GPU's quality settings @ 1920x1080. Anand Bench shows some Portal 2 benchmarks
The HD 7770 is a good fit. The HD 7850 gives some headroom. Nvidia hasn't released their GTX 6xx mid-range cards, yet. The GTX 560 Ti is the best fit from Nvidia.
Tribes Ascend is a bit more demanding. Techspot has benchmarked
the game at very high quality settings. Here, you'd have to dial down your resolution and quality levels for the 7750. The 7850 would allow native resolution and very high quality levels. On the Nvidia side, it's the GTX 560 Ti again.
So, it's more than $200 if you don't want to dial down quality and/or resolution settings.
7770: Sapphire has shown off their passive 7770 Ultimate, but it's not available, yet. Their Vapor-X version is pretty quiet. ($150)
7850: The MSI R7850 Twin Frozr III and R7850 PE Twin Frozr IV have gotten good reviews and feedback. (~$260-280)
GTX 560 Ti: I haven't been tracking these...upside is they are in the $200-220 range, downside is they burn 50W more power than the 7850 while gaming. That heat has to be dissipated. So, similar cooling solutions will be louder for it than the 7850.
Hope that helps.