It depends on your resolution really. What size of screen are you going to be gaming on? I don't have a 4670, but I've got a 9600gt myself, which when you look at benchmarks and what not, the 4670 and 9600gt trade punches, but the 9600gt may be like 1/2 a step ahead in some things.
If you are playing on lower resolutions like say maybe on a 19 inch or 20 inch monitor, you should be ok. I think it will actually perform well for you. My 9600gt runs pretty much everything I want it to. Keep in mind too, your video card will be the slowest part of your new computer.
In all seriousness, I'm running an Athlon x2 5200+, so quite a bit slower than your i5, and 2 gb of ddr2 memory. With a 9600gt, my rig still gives decent performance on most titles. I imagine you will get great performance but you just may have turn down some details and turn off AA on some games. The 4670 is a good card, it's the cheapest gamer card I would reccomend, but if you are playing lower resolutions, and with the rest of the components being as strong as they are, I think you will be happy. And you can always upgrade later. It will probably do ok on WOW, but don't expect a lot on crysis, it should run ok, but you'll probably have to turn some things down, but keep in mind that game can leave even the best systems gasping for air.