RAID 1 not performing as expected

Windows 7 (and friends) seem to make use of some fairly aggressive software write caching, which on my Windows Server 2008 R2 workstation (using two RAID-1'd drives), causes all sorts of wild memory consumption fluctuation during large sequential I/O operations. This tends to skew drive write speed benchmarks a fair bit, so you might want to turn it off for before/after comparisons.

To do so, you would go to Control Panel -> Device Manager, expand the Disk drives column, go to the Properties for your RAID-1 volume, and under the Policies tab, disable both write caching options. (The second should not be set on your onboard Intel controller unless you've got a UPS, and even then I wouldn't trust it.) Rerun your benchmarks and see if your results are as odd.

Aside from that, have you benchmarked just the new drive on it's own? I have seen instances where a slow drive and a fast drive RAID'd will "meet in the middle" performance-wise; your new drive may be quite a bit faster than your old one, and the latter bottlenecking the former read-speed wise. (Intel's software-y fakeraid is almost definitely prone to this; dedicated hardware controllers will probably handle split reads of dissimilar-performing drives better.)

You'd only see the increased read speed if the RAID controller will actually split incoming read requests across multiple drives. If you're using Intel's chipset RAID, I'm not sure that it actually does this.