Upgrading from 4890, about 300$ budget

I'm planning on getting a new GPU for my birthday in about 2 weeks and was wondering what would be the best choice with a budget of roughly 300$ but it's not rigid.

I'm currently gaming at 1920*1200 and mostly playing WoW (which completely made my 4890 whine like a baby in outlands with everything on max), the S.T.A.L.K.E.R. series, might get metro 2033 and also planning on getting black ops.

I currently own an XFX 4890 ZSFC (the stock one at 850mhz) and a Silverstone strider 750 watt modular PSU (not the plus series, just the regular ST-75F)

Mobo is an EVGA P55 FTW which cannot Xfire at dual 16x... only 8x
CPU is an i5 - 750 @ 3.96ghz (Yeah I know, not 4ghz but no matter what I tried, It doesn't like this number at all...)
CPU cooler is an NH D-14
RAM is 4gb of g.skill ripjaws 1600mhz
HDD 2*250gb WD caviar blue in RAID0 and 1*1TB caviar green
CASE is an HAF 932

I will probably try to OC the new card although it is not really a prime point in my decision. As for Xfire, I am also considering it.

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I was thinking about getting a 6870 (since it looks like great value from what I read) now and trying to get my hands on another one somewhere in February for Xfire since they scale like crazy and it would still give me a nice upgrade as single GPU from my now aging 4890. The 2 concerns I have about crossfire is the very probable bottleneck because of the dual 8x mobo and the power requirements which are at 700w according to this : http://extreme.outervision.com/psucalculatorlite.jsp

I also considered the MSI 470GTX TwinFrozr and XFX 5850.

What would you think would be the best choice ? one 6870 with crossfire down the road, GTX470 or XFX 5850 ? Feel free to suggest anything else, I'm open to ideas.

I would also prefer a Canadian site and also, I've no credit card so I need to pay in another way.

TIA,