Nvidia doesn't exactly have a direct competitor for the 7770. The GTX 650 is roughly the same price as the 7770, but in terms of performance it is more often competitive with the Radeon HD 7750. The GTX 650 only really competes with the 7770 performance wise in games that very heavily favour Nvidia, eg. Blizzard titles.
The GTX 650Ti is a bit faster than the 7770, but also more expensive, you can find 7770s around $120 or so, the 650Ti is priced at $150 or higher.
Depending on how much you are willing to spend, the 7770 may be a better buy. The GTX 650 really isn't powerful enough to take advantage of any of Nvidia's proprietary technologies, and it will be a weaker performer unless you are buying this card mainly to play World of Warcraft, or Starcraft II.