Card for new build; need good single performance and SLI/CF for future

APPROXIMATE PURCHASE DATE: Sometime this week

BUDGET RANGE: $300 before rebates (must match or beat card I'm looking at)

USAGE FROM MOST TO LEAST IMPORTANT: Gaming first, and of course, I want to be able to handle the big one -- Crysis at 1920X1080 with fairly high settings and/or high AA.

CURRENT GPU AND POWER SUPPLY: No GPU atm since it is a new build, PSU will be a Corsair 750w.

OTHER RELEVANT SYSTEM SPECS: Will be paired with AMD Phenom II X4 945, mobo is up in air -- can get one of these through a Newegg combo deal with my CPU, but can change if SLI cards are recommended/better mobos are suggested; Case will be a Cooler Master HAF 922

PREFERRED WEBSITE(S) FOR PARTS: Newegg.

COUNTRY OF ORIGIN: United States

PARTS PREFERENCES: Would prefer ATI

OVERCLOCKING: Maybe

SLI OR CROSSFIRE: Yes, in the future

MONITOR RESOLUTION: 1920x1080

ADDITIONAL COMMENTS: My thread title may be a bit confusing, so let me elaborate: I want to get a single card for a build I'm working on, one that has good to great single performance, but will also be relatively future-proofed via Crossfire or SLI (that is to say, if I get another one sometime in the future, a dual/multi setup will allow it to keep up with the then-high-end GPUs. Right now, I am partial to a 5850, as for $100 less than its bigger brother, it still has amazing performance; indeed, videocardbenchmark.net has it ranked 5th overall, and on Crysis, it's been benchmarked as performing BETTER than a GTX 470 despite its VCB rating being lower (its sample size is also much bigger on VCB, so it's likely saturated.)

Now, while $300 can buy me one of these supercards, I'm reticent to do so. This is my first gaming build, you see, and I am not particularly thrilled that, as projected, a quarter of my budget goes into my video card (and yes, I know that's the idea of a gaming rig, but I just want to know if there are better frame/$ ratios out there). I was playing with the idea of twin 5770s, one of which is half the price of a 5850... except that two 5770s about match the 5850's single performance, and that would be the limit of my upgradeability given the >2 card Crossfire scaling.

So, bearing in mind that I haven't much looked at Nvidia cards, or if you have ATI cards that I missed, are there any cards that would be similar in their current-gen performance to the 5850, and in their ability to perform in dual-crossfire setups in the future?