AUDIO interface = does it help or not? Options

[riket]

Hello.

Could someone help me understand who I must believe?
My config : iMac DV 400 / 512Ram., Cubase 4.1, electric guitar in audio from Roland GS6.

The scenario :
As I use PPG vsti together with audio guitar lines + other audios (vsti's like Sample Tank piano or other vsti's) my CPU hits the overload, audio cuts off, comes back after I disable one vsit or two to unstuck the stuff.
How can I improve the CPU performance without changing of Mac?
Is an external audio (UA-5 for ex.) a solution (attention, I mean while PLAYING PPG in live processing, not as printed lines, because with many printed audio tracks I have no problem, of course)??  
I have asked already 10 different dealers in Belgium, 50% say it IS the solution, 50% say it doesn't help at all in this case cause what you send to the extern. audio is ALSO processed by yr cpu, thus as much work for it than with an internal audio : it moves the problem to another place but won't help.
The others say the dsp of UA-5 will take so much in charge that yr CPU will work seriously faster, while the processing of the PPG play occurs on the external card, they say.
Does anyone have any experience with a configuration close to mine?
Thank you for any input.
riket