Actually, I bought an ASUS U3S6 plx bridge some time ago. This could in principle deliver 6GB/s sata, and can be used for boot disks. I have not gotten it to work properly though...
Do you really think I can add two years to the rig by adding a 780? Then that would be a good sollution to me. Would my system bottleneck the 780 (e.g.http://www.newegg.com/Product/Product.aspic?Item=N82E16814121779
mouse24: according to the graphics roundup for April on this page, the 7850 is two tiers above the 5850... But still, you are probably right that two 5850s are stronger than one 750ti.