Wow, that's the first time I even looked at the age of people who posted, never noticed how many people under were here... I mean, I knew there were some, not that many... and guess what... I don't care, if they can think rationally which most(I say most cuz I assume that some can't) of them can, then I'd much rather follow their advice than someone who can't figure out that buying 32% of a card(cuz that's the equivalent of what it is) isn't the deal of the year.
I mean, sure enough there are people who make/have lots of money, not saying I do or I don't, but when we give advice, we tend to think about maximising everything in regards of the money spent.
As for SLI, it is not worth it because the performance gains do not justify the extra money spent. The extra cost isn't only calculated with the GPU, but also the mobo, which needs to be able to take it, the PSU which needs to be able to power it... Then it adds more heat to the case, so the extra needs to cool down... And obviously the extra needs to power it. So, IMHO, it is not worth it for your wallet and the environnement.
It would make more sense, to actually buy a high end card now and buy a brand new one when you need to, the gain in performance will be more significant and it will last longer. Now, of course there are some cases where SLI is useful, but given the little info you gave us, one could not possibly assume you are in that situation.
Information we know about you:
You have a limited budget of 100-175$
You play games
You are not filty rich (assumed from the fact you are thinking about selling your old parts)