DPC latency (that old chestnut!)

Hi there everyone. I have a few questions about latency.
I've googled the net to death and there seem to be lots of conflicting ideas out there.
 
I run XP on a desktop Intel Duo @2600Ghz with 2 GIG ram. Sonar 2.2 (yes it's old) lots and lots of VST use and audio doesn't phase the system at all.
I'm thinking of buying a laptop for portability and have downloaded DPC checker to take on a stick to hunt down the laptop.
Also thinking of dual booting XP so that my 'ancient' gear can still run, because there are issues for drivers on Windows 7.
 
I've tried DPC latency checker on my desktop so I know what I'm aiming at.
Taking off all unnecessary services and with no programs running, gives around 5us which seems very good.
Loading and playing a Sonar track gives around 150us with no spikes.
 
DPC checker info doesn't seem to say if you should check under load (with Sonar running) or not, in order to check the latency. Which is correct?
Clearly I can't load Sonar on a laptop in a store and run it with DPC, so will checking with DPC even give me any idea how it would perform?
 
I run an Edirol UA-101 via USB 2.0 and I'm not sure the latency will even be an issue given it's not an on board card. (There seem to be a lot of different ideas around this). Anyone got a definitive answer?
 
I may end up upgrading Sonar at some point too, so any help with what other laptops people are using and finding ok would be great.
I'd like a custom machine like those from JR, but the £ is not so good against the $ (or any other currency come to think of it) at the moment!
 
Many thanks
 
Steve