Though I usually always agree with a single card, I will provide a counter argument
I think when it comes to high end options for GPUs, there are really three.
1) A person without a GPU or with a very dated one buys two high end cards for top-of-the-line (and probably overkill) performance (i.e. two 7950s or 7970s).
2) A person without a GPU or with a very dated one buys a single high end card (7950/70) and leaves an upgrade path a year or two down the road in the form of adding a second card.
3) A person with a recently (within the last year or so) top-of-the-line GPU (6950/70) adds a second card in Crossfire (or SLI if we're talking about nvidia) and attains the same performance or better as a single, currently-top-of-the-line, GPU (7950/70).
I think that your situation, Heatmiser, fits this third description. Unless you are against dual-card configurations (which is perfectly justifiable given the hiccups typically associated with them), the best bang-for-your-buck option is definitely adding a second card in Crossfire. You'll have better performance than a 7950 and for probably $150 less.
I do like my 7950 and, full disclosure, I upgraded from a 6950, so I am going against my own advice here, but I only did so because I sold my 6950 for the same price I ended up paying for the 7950. Unless you're getting some anomalous deal on the next gen card, I think the upgrade path which makes most sense is...
Single, high-end GPU...(wait a year)...Matching, high-end GPU...(wait a year)...repeat the previous two steps...
Just my two cents
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