Can you mine both Bitcoin and Litecoin at the same time on a computer with a GPU/GPUs?
On each individual GPU, it's only possible to mine one coin at a time. Although I haven't tried it, I believe it would be possible to mine Bitcoin with one GPU and Litecoin with another GPU in a single machine with two GPUs by running two instances of cgminer, but I don't think this is what you're looking for. There are a few coins that can be merge mined (mined simultaneously) with Bitcoin, like Namecoin, but they're not worth much.
As for recommendations on a 7950, I'd throw in a vote for Sapphire. I think the general consensus on the Sapphire cards is that they're good performers (but probably not the best), run cool and provide maximum tweakability (can be undervolted). I have 3 Sapphire 100352-3L 7950s and they've served me very well (620Kh/s each (Litecoin), 1072Mhz core, 1250Mhz memory, 1080mV core - temperatures are 64C core, 75C VRMs in an air-conditioned room). These cards have the "Dual-X" cooler with the big heatsink and dual fans. This keeps the card much cooler than the reference heatsink/fan design. As realtime pointed out, the reference design can still be useful, however, if you're trying to fit many cards directly onto a motherboard. I wouldn't recommend such a setup though -- just use (powered) risers.
Gigabyte and MSI 7950s seem to perform well too, but my understanding is that most Gigabyte cards are voltage-locked. I wish I had some experience with them to be able to tell you more, sorry. XFX cards seem to be all around bad for mining. I see so many complaints about them: too hot, low hash rate, etc. Powercolor cards seem hit or miss.
Whatever you choose, good luck!
