Two 9600GTs in SLI. Generally the 2900XT will give you performance a little above a Radeon 4670 or 3870, but a bit below a single 9600GT in just about everything except for the few instances where the insane 512bit bi-directional ring bus gives it an advantage, which is usually where it's unplayable anyway. Go with the two 9600GTs.
9600GT's are crap for gaming. Id consider other options like 4830's or 8800GT's.
Absolutely not true.
two gt9600 in sli would give you a very good performance. Check web for benchmarks.
So if you already have a gt9600 you can get another one for very cheap. If you are building a new system then go with something newer, depending on your budget hd4850 or hd4870 will give you nice performance/price ratio