Just looking at the hierarchy charts, if the 9800 SLIs as good as the 4850 Crossfires, then going that route would put you on par with a 560Ti. Only downside is you might get micro-stuttering which you can read about in Tom's article. For $70 it is probably worth it, but you also won't have DX11 support.
For $140, you can do slightly better than the 550Ti. I've seen the 6870 as low as $140 on Amazon after rebate (granted that sold out fairly fast), and 6850's are generally around $140 and you can sometimes get the factory overclocked ones at that price.
Not really intending this rig to ever support DX 11. Just wanted to prolong the gaming lifespan of the system. Also it is better when I eventually hand the system down.
For $70, I would be getting another EVGA, 9800GTX + and an SLI Bridge for Free. It seems like a much better value to get SLI Performance which is till better than a single budget card like the 550Ti. The "+" means it is already factory overclocked or Super-clocked according to EVGA.