Should I get another GTX 275?

What resolution?

2xGTX275 will keep you happy for years to come probably.

Not 100% sure about your PSU's ability to handle it though.

I'm in the same boat actually. Very similar system too. Kind of wondering if it would be better to just add another GTX 275 or wait for GT300 cards.

My system is:

Antec 900
Core i7 920 (2.66GHz Overclocked to 3.35GHz on stock fan/heatsink)
3GB (3x1GB) Corsair XMS3 DDR3-1600
eVGA X58 3xSLI LE mobo
eVGA GeForce GTX 275 896MB (x1)
500GB WD Caviar Black 32MB Cache
700w OCZ GameXStream LED p/s

TBH I personally think 2xGTX 275's would be the way to go. When you think about it, the new GT300, unless they really pull out all the stops, is going to be about 1.6x to 2x the performance of the fastest single GPU GeForce (GTX 285). All that for about (presumably, comparing to AMD's new 5870 pricetag) $399. Whereas you can get a second GTX 275 and get the same if not better performance than a single GT300 GPU for only $210. About half the price for the same performance, assuming scaling is working properly. Well thats my 2 cents at least.

PS - your power supply is fine. an Antec TruePower 550w can handle 2xGTX 275 in sli. It even says so on the nvidia website. So having any power supply that can provide continuous power of more than 550w is more than enough.