Captivate is a good phone but no idea when it will be getting 2.2 or higher (without voiding the warranty).
Many rumors are suggesting that a Nexus S with AT&T 3G bands will be coming out in March (similar to how the Nexus One was released, first with T-Mobile/Euro/Asia 3G bands and later that quarter, one with AT&T 3G bands)-
Mobilicity Says Nexus S Heading to Canada’s Big Carriers; Probably in March | Android Phone Fans but I highly doubt that it will be available under contract (due to lack of bloatware and AT&T's restrictions, ie. side-loading non-market apps). Alot of this is speculation so time shall tell.
As far as on contract alternatives go, there were 3 high end Android devices for AT&T (with full support for HSPA+ and all with Android 2.2) announced at CES a couple of weeks ago:
-Samsung Infuse (a Captivate with a 1.2GHz processor, 4.5" screen, front facing camera and Android 2.2),
-HTC Inspire (like an Evo without a font facing camera, 768MB of RAM instead of 512MB and featuring the newer Qualcomm Snapdragon platform which is better in terms of graphics)
-Motorola Atrix (in terms of pure specs, it probably is going to be the most powerful phone at the time of launch. Features a dual-core 1GHz processor, 1GB of RAM, 4" 960x540 screen, front facing camera. Only problem is that it has a locked bootloader similar to your Backflip making it hard to load custom kernels if you are into the modding scene).
No idea when any of these 3 phones will be releasing but all are rumored for quarter one of this year. If you can hold onto your Backflip for a couple of months, it might be a good idea.