At this point in time, the replacement for the GeForce 7900GT is going to be called the 8800GT. Its performance lies somewhere between the 8600GT and the 8800GTS and will be at the same price point your current card was when it debuted. Note that it has only been recently announced and may not be available in your area for a few weeks. You can go for the cheaper 8600GT and still get the same peformance you are getting now, but with the added benefit of support for DirectX 10 games. Unfortunately, this card will be living on borrowed time, as far as new game titles go.
Of course, you also have the choice of ATI to upgrade your system with, but at this time I don't recommend ATI branded video cards, due to software and hardware issues peculiar to this brand which, despite being years old, still have not been addressed. Let me put it this way: Unless you're getting it for free, you will be disappointed.
Here are the top 3 performing cards:
8800 Ultra <- ridiculously expensive, exactly the same as 8800GTX but clocked faster. I haven't seen one for less than $600.
8800GTX <- Flagship card of the 8800 series, for serious gamers and graphics pros only. These cards cost over $500.
8800GTS <- Originally the bottom of the line, these are highly recommended for game enthusiasts, but are overkill for non-gamers. These cost around $350.
They represent the "top end". The next 3 are "middle tier":
8800GT <- Designed for the casual gamer, this is probably the one I'd recommend to you. It will offer a significant performance gain yet should still be available for under $250 at launch.
8600GT <- Probably equivalent in performance to the 7900GT, but supports DirectX 10. For the budget gamer. These run at around $175 currently.
8500GT <- Allows you to run with all effects in Vista, and is DirectX 10 compliant, but not a great performer. This one runs slower than your current model. These can be had for under $150.
There are a few other even lower end Geforce 8 series but these are for OEM machines and I wouldn't recommend any of them.
Here is a link to a comparison chart:
http://en.wikipedia.org/wiki/Comparison_of_NVIDIA_Graphics_Processing_Units#GeForce_8_seriesP.S. I should also mention, the cards *will* draw more power than your old card did. I recommend that you have at least a 500W power supply if you purchase anything better than the 8500GT. For the 8800GTX/Ultra a 650W would be even more advisable. This is partly due to the fact that power supplies lose output over time and will degrade by as much as 30 percent.
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