I benchmarked the m4's one at a time with As-ssd before RAIDing them together, I'll post that info as well. I'm glad I did, because I was accidentally using one of the m4's with an sata II port originally.
I think that you'll probably still notice the jump up to a single m4, but keep in mind that your 160gb g2 is faster than my 80gb g2. Plus, I was running SRT with 25% of my drive, so it was actually extremely slow even for an older ssd. And I didn't install windows onto the m4's individually, so I'm only guessing at how it would be on a single drive. Anyway, I've got the m4's individually, one of which I inadvertently measured on sata II as mentioned, plus I have them both with the 0309 firmware in RAID-0 that they shipped with and with the latest firmware in RAID 0. I should be home in 30 minutes or so, I'll throw all of the screenies up then.
I will say this for sure. Regardless of what the benchmarks say, and the benchmarks look great btw, I am even more impressed than I expected to be. I am absolutely just as happy with this ssd upgrade as I was with my original g2 80gb upgrade, and that was the best/most valuable single upgrade I had made in probably a decade. You might go to buy.com and grab the single m4 for $190, then if you decide that you want more oomph you could always grab a 2nd one to really ramp things up. Keep in mind the amount that you can sell your x25m for OR buy another one for, however, because 2 x x25m 160gb in RAID 0 would probably be faster than a single 256gb m4 and also have more space. My 80gb drive was just too small for it to be worth it for me to RAID a couple together.