Well you really don't need to upgrade, Are you planning to put the 5770 in the HTPC and want to upgrade yours for something that will last a bit longer at the same resolution ?
If so depending on the time frame for needing the HTPC up and running i would tend towards waiting for Crysis 2 benchmark tests to come out and see what the various levels of cards bring to the game. As an example i have played the original Crysis on a X1650 XT and a 2600XT but when i played it on a 3850 it was like a different game visually.
This could be the case with this game, something like a 6850 may have the ability to enable effects the 5770 cant but thats all guess work until reviews come out.
It may be that just getting a low end HTPC card like a 5550 now for teh HTPC and then getting your upgrade when you know whats what would be the better option. There will be a good size market for second hand 5770's so selling yours to off set the cost of a new card is a valid option as well.
The integrated graphics of a sandy bridge cpu should be good for a HTPC; you probably won't have a need for a discrete graphics card.
To help assess your current needs, run these two tests:
1) Run your games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics card.
2) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.
It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.