I had a viper 7900 installed in my 05 truck but had a lot of issues with range and the remote losing it's pairing. I went back and had the 7900 ripped out and a Viper 5900 installed. The 5900 is an alarm with a built-in remote start. The 2 way remote is fantastic with range over 1.5 mile. It works everytime with no issues at all.
If you decide to install the 5900, make sure that the installer uses BypassKits to interface it to your vehicle that way you can still retain use of your factory remotes and not affect anything with OnStar, BypassKits (http://www.bypasskit.com/categories.aspx?catid=3
) works very very well with GM unlike some of the other products that I've heard about. My setup includes the 5900 (SST) with the BypassKits and no complaints.
Best of luck, let me know if you have any questions.