Definitely stick with the one card. At 1080p Resolution a 7950 or 7970 will max out pretty much any game.
This is a build I made on IBuyPower's website, which I can recommend as I've bought two system from them for family members. You're going to be paying a premium though to have it built. Do you have any local computer stores near you? You mentioned you cannot put it together yourself, but I would definitely check around to see if any of them would put it together for you. I'm not sure what your disability entails, but this would allow you to get exactly the parts you want. Shops around my area charge $50-100 for this. Honestly, the markup online isn't that much anymore. Comparing the build that I made on IBP and ordering the actual parts, IBP was actually
cheaper than ordering the parts and assembling it yourself. I couldn't get exact specifications on some of the parts, but here's the PP link for the identical build.
http://pcpartpicker.com/p/znv4 After shipping, you're looking at basically the same price.
Edit: If I were going to go with a build right now, this is what I would go with;
http://pcpartpicker.com/p/znBr
I'm using a Gigabyte 7950 currently, clocked up to 1100/1400 from stock clocks 900/1250, which should put it over the performance of a Stock 7970 such as the one you're getting with the previous build I mentioned. This build includes a modular PSU, a 128GB SSD, and a 1TB WD black instead of a Hitachi which have a reputation of failure before they should.