Good question, but I don't believe they would do that until WoD is actually announced for release.
I'll say this, and take no offense but, if you have to wonder about requirements, its time for an upgrade. I was able to put this monster computer I'm using atm with Syiler's help for less than 850 USD before Operating system etc. You can EASILY build a 500 USD pc that will run every game you play on ultra without issue.
If you are still concerned about specifics, I'd say just examine MoP's. If you are right on the line with MoP, WoD will likely increase them incrementally, so its probably to expect some issue.
Best of luck though, I'd really recommend a new machine. The graphics and settings of WoD are not something I'd want to see on anything other than Ultra!