In general, video card upgrade should be preferable as it should boost speed much more seriously.
Your existing card (4850) is not that bad as compared to 4890. The difference between single 4890 and single 4850 should be roughly the same as in games (that is, around 30-40%). Here are some useful Left4Dead benchmarks:
http://www.techreport.com/articles.x/16681/7For single-GPU video cards, performance difference in iPi Studio should be roughly the same as performance difference in Left4Dead. You can use this fact when choosing video card.
By upgrading CPU from Athlon 64 x2 6400 to Phenome II in Ipi Studio you can only squeeze some 10% improvement at best.
We plan multi-GPU support but it is not ready yet. It may take a month or two before we come up with multi-GPU support. We expect iPi Studio performance to scale almost linearly with multiple GPUs.