The GTX 560 Ti will outperform the 6870 in most scenarios (though the 6870 outperforms a base GTX 560, all day long); a GTX 560 Ti 448-core is an even better performer, but the price is noticeable.
Hello Operator brings up an excellent point, though: The 6870 is highly-affordable and can run pretty much anything that you'll be playing. I can't speak to the performance of Rome 2 since we haven't yet been given an opportunity to benchmark it, but most other games won't have a problem. Metro might beat it up a little - Battlefield 3 might be run on high instead of max, but in the end, you're saving a few bucks and the loss in performance is minimal and often game-specific.
The great thing about video cards is that they're extremely modular, which makes them easy to upgrade later. You want a strong core system (motherboard, CPU, SSD) that you can rely on for years, and after building this, you can continue upgrading the GPU when gaming advancements dictates.
To sum things up: I highly encourage an SSD purchase and, if the SSD runs you over budget, definitely don't feel bad about a drop to the 6870.
One thing I want to ask: Why the 2500k? You could get the newer 3570k for $10 more and it will perform a little better. It also has a strong IGP (integrated graphics processor) in the event you ever need to live without a GPU. Here's a link to that CPU:
www.newegg.com/Product/Product.aspx?Item=N82E16819116504
It's worth considering, but if $10 will break the bank, it's not worth going over.