It's custom built.
Before I say anything else I want to make sure that your problem is your GPU and not your CPU.
Pick a game (recommended: BF3/4 or Crysis 2/3) that is very demanding on performance and adjust the settings until you can just barely enter a playable state of the game (i.e. 30+ fps with no drops). Once you are at that point, push the settings a bit higher and start playing. After you get into the gameplay and things start to lag, minimize the game and open task manager - switch to the CPU tab. If the usage graph shows your CPU was running at a constant 90-100% load at any time while you were playing, then your bottleneck is your CPU. If the load did not hit 100% but you were still lagging, then your GPU is the bottleneck because it can't handle the load.
In my case if I play BF4 on ultra my CPU never exceeds about 60-75% load, which means I could get better performance by upgrading my GPU (the CPU could still take 25-40% more load before it became the bottleneck).
So assuming your bottleneck is your GPU, keep reading. If it's your CPU, buy a $350 3.7Ghz i7 4280k and then re-evaluate your performance and bottleneck.
Continuing on with the GPU discussion...
Are you sure that the motherboard in your desktop has SLI/Crossfire support? - Look it up.
Not only does it need to have two full PCI slots, but ideally both should be PCI-x16 and meet PCI-Express v3.0 standards. Depending on how old your build is, your board might only be PCI-x8 + PCI-x4 while in SLI/Crossfire (which means PCI-x16 only when using a single slot) which limits your choices as far as how new you can go with your cards. If your build came with a 7900 in it, it
should have a new enough board to do dual PCI-x8 SLI/Crossfire.
As for the card name you provided, "AMD Radeon 7900" is just the chipset series of your card - the last two digits denote which model it actually is, for example 7970. The 7900 series is still relatively new but you can do better as far as performance at a reasonable price.
Here's a list of the three available models in that series:
http://www.amd.com/US/PRODUCTS/DESKTOP/GRAPHICS/7000/7900/Pages/radeon-7900.aspx#2Depending on if you bought an aftermarket version such as MSI or Gigabyte, your card my have extra ram or better cooling abilities than the standard models on that page.
Plan to sell your current 79xx card in order to make at least $120 (depending on the model and brand) towards a better Nvidia card. Plan to spend at least $250 + tax and shipping. Whenever you find a model that you like, look to see if there is a superclocked version, and in addition look to see if there is a superclocked version with extra ram. If you can splurge the extra $$ to get either of those editions, you will have a card that will last a lot longer than your AMD 7900 would on next-gen titles.
Once you have purchased that card, your new bottleneck will be your CPU.
Here is a benchmark reference chart for high-end GPUs:
http://www.videocardbenchmark.net/high_end_gpus.htmlUse it to directly compare cards to narrow down your decision.