I wouldn't go with anything less than the HD 6770. It's an excellent entry level gaming GPU, and is priced appropriately. The HD 6770 is simply a re-badge of the HD 5770, a great card for mid-range system. I actually have an HD 5770 in my secondary PC and it does very well on most modern games, including NBA2K11. It will play most other games, such as Crysis, Battlefield Bad Company 2, Assassin's Creed 1 & 2, Far Cry 2, Half Life 2, Team Fortress 2, etc... etc... on medium to high settings as well at a resolution of either 1680 x 1050 or 1920 x 1080.
I can play NBA 2K11 @ all High settings, except player detail on medium (due to cloth physics) with my secondary PC that has the HD 5770. This also includes a rather heavily modded game. FPS is in the low to mid 40's, which is very smooth for this game. The reason I cannot max out the Player Detail is due to a weaker processor that's not up to the task of handling the cloth physics. The CPU is a Athlon 64 x2 4200 @ 2.5 Ghz. A very dated CPU. However you have a stronger, 2nd generation, quad-core, processor and will likely be able to max the game out. I expect 2K12 to have very similar, if not exact, system requirements as 2K11, since it's
apparently running on the exact same engine.
I will recommend that you get the
XFX HD6770 from Newegg.com. XFX is a great company and have excellent double-lifetime warranties for many of their products if you simply register them online. I've owned a handful of XFX products over the years and never had issues. I currently have the previously mentioned XFX HD5770 in my secondary PC, and an XFX HD6970 in my main gaming rig. Both are excellent cards for their targeted audience.
Oh, almost forgot. Your Corsair 450w PSU is more than sufficient for the HD6770. You really only need a quality 400w, so you're good. Corsair is another great company and make top-notch PSUs. You were spot on with the amperage too; it's 33A on the +12, and 20A on the 3v and 5v rails, which is solid.
I Hope this helps and if you have any other questions don't be afraid to ask.