To my experience the latency of a device setup does not change, it is just how long it takes for the sound to come out of the monitors. Your problem seems a lot more like a RAM or processor problem. What is your setup? The version of windows, your processor, and your ram, i mean.
I run a q6600 intel core 2 quad with 6 gb of ram (rarely will you need 6) and it handles up to 30 softsynths, my guitar rig, and ezdrummer with no problem. I also run Sonar on Windows XP 64 bit. I have found that if your setup can work seemlessly on XP 64 bit (meaning there are no driver issues, etc) it is the best way to go, it gives your programs much more power without windows eating up your processor and ram. Just something to consider.
I would suggest 4 gigabytes of ram, and a core 2 duo or quad, since you are using plugins.