What resolution are you planing on gaming at and what games are you planing on playing?
Also, please post your full system specs including your PSU brand, wattage and +12v amperage.
If it is necessary for you to get a more powerful setup than a single 4890 1Gb, there are much more cost effective options than a 4870x2.
Either CrossFire 4870 1Gb's or SLI GTX 260's will give you just as much gaming power as the x2 and cost quite a bit less.
Also keep in mind, if you can get away with a single 4870/GTX 260 for now, DX11 GPU's will be out in the next few months.