The reviews are pretty split with some sites favoring one and others the other....but the 6850's performance seems to sit between the 768 MB and 1 GB versions of the 460. here's one review favoring the green team.
http://www.hardwaresecrets.com/article/XFX-Radeon-HD-6850-Video-Card-Review/1127/12
....the GeForce GTX 460 with 1 GB was up to 32% faster than the Radeon HD 6850 (between 8% and 32% faster on 3DMark Vantage, up to 10% faster on Call of Duty 4, between 7% and 18% faster on Far Cry 2, between 5% and 9% faster on Alien vs. Predator, up to 11% faster on Crysis Warhead, and up to 15% faster on Lost Planet 2).
Other sites have different benchies and go the other way.
I like benchmarkreviews articles cause they include a value analysis which I'll paste in here:
http://benchmarkreviews.com/index.php?option=com_content&task=view&id=627&Itemid=72&limit=1&limitstart=14
GTX 460 768 MB - $57.05 per fps
Radeon 6850 - $57.40
GTX 460 1 GB - $68.57
Keep in mind that today's prices will change those numbers even if the article is only a week or two old. A lot will depend on "buy day" prices.....
So, it seems to be a neck and neck thing and in that case, I'd have to lean nVidia. The 460 comes with CUDA, PhysX, etc. The fact that you already have a 250 which could be kept and used as a PhysX card would seal the deal for me.
Looking at newegg I see among "brands that I'd buy", both the 1 GB 460 and the 6850 at the same $200 price. At $200 for either, that would make my decision easier to go with the 460, especially if ya can find a superclocked one at that price like this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130571