Hi,
The lower speed of the RAM, in this case, won't cause you any issues. However having the extra RAM on-board will allow you to use more of the Plug-ins and Virtual synths within Sonar and will really help when you come to do your mixing if you need to use a lot of effects.
I am using a Delta 1010 and Cubase 5 and have done a lot of live multi-track recordings and the only real problem I had was when I set the audio buffer size too small when trying to improve latency.
Good luck and if you run into any recording snags, feel free to e-mail me at my Hotmail account
ATB