Not worth it
I was running a couple of 9800s for a while until one died, but that's not the problem. The problem is that you can get a GTX 460 these days for around $150 AR. Both the 768mb and 1gb 460s run better than both 9800s on SLI. And also, two 512mb cards running in SLI does not equal 1gb. Games that require a lot of video memory like Grand Theft Auto 4 will suffer (although, that's the only game so far that I've had that issue with). The 9800 does not have DirectX 11, which isn't a big deal right now, but what about next year? It's also a dead end to get two older cards as an upgrade; if you want to further upgrade, you'd have to get rid of both cards. I'd say just save up another $50 and get a GTX 460, at least, and when you want a bigger upgrade, just pick up another one and run those SLI. I ended up getting an XFX ATI 5850 1gb for $180 AR around Thanksgiving and am running my remaining 9800 as a dedicated physics card (with a super easy tweak that can be found on TomsHardware.com in the article "Analysis: PhysX On Systems With AMD Graphics Cards").
UPDATE: Speaking of that NewEgg has the EVGA 460 768 and 1gb versions for $140 and $160, respectively. If going for the 1gb, I'd spring for the the model 01G-P3-1373-AR to get a lifetime warranty ($180 AR). Make sure to register your card right away (that's why I had to buy a new card instead of sending it in for a warranty repair)
