I normally recommend one card, but since you have a 5850 and can get another for ~$100 ..... I would do it.
A cheap upgrade that will last you another year, and then you can look at the amd 7xxx and nvda 6xx's.
1) Buy a 5850 and crossfire
2) Sell my 5850 for 100-120$, keep my 100-120$ and buy a 240$ video card. I can have a 6950 with 1Gb ram for this price.
When I look at the benchmark on anandtech however, the 5850cf is always better. I'm just wondering if the watts/temps/noise level will be that worse to make me regret going CF. I got a good quality case in Antec Mini P180 ( 120mm+200mm top air exit) + 2 120mm Noctua fans in front for air input, and you can probably guess that I like quiet computers with my seasonic x-650, noctua fans, antec mini p180 case, etc
I'm still not set. But 100$ for 5850x2looks so good on paper!
there are 2 issues with crossfire
1 - microstuttering, not all people experience it, but if you have it it can ruin the experience
2 - game support - when most new games are released they either dont support crossfire or have poor performance until the first patch or AMD driver hotfix/application profile
If it were me, id go crossfire, try it for a while, and sell both cards if you dont like it and get a 6950. If you dont like stuffing round and just want games to run right then maybe a single card is better for you.