that is a giant preference arguement, but I personally like the most powerful I can afford, that's why I have the card I do currently. I almost ended up with the ATI 5970, but they were out of stock, so point being: I like well made cards, not an affinity to one camp or the other. In the older AMD/ATI cards, crossfire didn't scale as well as SLI, so any of the 5000 series or older were beaten by Nvidia SLI setups. It looks as though what you'll be able to afford and get the most power from would be the Nvidia GTS 250. When you add a second card your combined set-up will work better than...well, there isn't anything in this price range to compare it to.
I'd say, if $80 is your ceiling, find the additional $20 you need to order the card and wait until you can SLI it. But this is really a rough decision, because that card is now 2 generations old, and not getting any younger. Patience might be your only option.