Based on the 1.5gb sounds like you have the GTX 660. The 670 will get you a faster GPU and higher frame rates. As for the video ram size an extra .5 gb won't make that big a difference. If you do proceed with the upgrade then make sure you get a card that is a reference design that blows hot air out the back of the card.
Yeah, if you want to future proof it, I would actually wait a bit till the 700 series is out. bf4 and the 700s should be out around the same time.
As for video ram, I would say 2gb should be a fine card unless you have plans to run triple displays.
I have a gtx650ti 1gb card in mine (stock was the 555) I run triple screens on bf3, the card can handle it on medium, but if I go up to high settings, then I get booted by punkbuster since my system cant keep up.
well if they dont there's always the full modular power supply route so yea lol, or i may just upgrade to a Aurora sometime next year but i dont see y they wouldn't up grad on power unless they think they can get away with the 660 being the "biggest" card they supposedly get away with for the next how ever many years it would be a bad business decision on there part.
I have a 670 FTW in mine and it works awesome. Putting it in was a bit of a task though with the cables. Tight fit but I managed. If you do decide to go that route, make sure that you have what you need. When I bought my 670, I had to buy some cables that plugged into the GPU since the stock cables have an "L" shaped bend to them and the 670 has a recessed plug in port.