I went ahead and bought Patriot LX 16 GB class 10 microSD. Now I get variable speeds from 3.5 mb/s to 8 mb/s depending on the file sizes. As I read somewhere this is due to fact that the phone is using MTP protocol to act as mass storage device.
Anyway with the old card I was topping only 2.5 mb/s no matter the file size, so it's definitely an upgrade and since class 6 was only 5$ cheaper I saw no reason not to buy the faster one, even if the milestone can't utilize its full capability. In the end its an old phone and I believe there weren't any class 10 cards out in 2009 anyway.
And one more thing, the camcorder isnt lagging anymore when recording videos
EDIT: After booting in recovery and clearing dalvik and cache partition I am getting 9.5 + mb/s, but it does start around 4.5 mb/s and then rises to its full speed. Cheers.