I would recommend that you try the 2x Vertex on your motherboard first. If you have a P45 chipset, you may get two of them to scale at near 100% for sequential read/write, it all depends on how ICHx has been implemented on your motherboard. If it doesn't scale to well regarding read/write speeds, check out the IOP performance using the two Iometer test files i made available and compare to my single drive.
For 4K random write test file, i was getting around 2500 IOPS and for the all in one test, i was getting around 1500 IOPS.
If you intend to use your Vertex RAID as an OS drive, then outright sequential read/write speed is not that important, and your motherboards onboard RAID could prove to be the simplest and most stable solution.
I have the Adaptec 2405 sas/sata RAID card. This should be good for upto 3x Vertex, if you must have 4x Vertex, then i would get the 5405, which has the faster 1.2GHz Dual Core ROC.
In each case, keep an eye on the IOP temperature of the RAID card, as SSD drives have been proved to absolutely hammer the IOP, and you may require some active cooling on the RAID cards IOP.
It would be great if you can post some results.
Regarding the Vertex's onboard cache, 64MB is no longer a rumour, OCZ confirmed that the production Vertex drives, from 30GB - 250GB all have 64MB of cache. The 120GB and 250GB models are still slightly faster, as the extra NAND flash in these models are able to access more channels on the Indilinx controller.