Perceptible latency when using software synths is history.

I am a Mac user so I can't make any specific recommendations. All I can say is:

If you are going to use a laptop use a Firewire audio interface. There are plenty to choose from.

This isn't something that you'd choose or decide on by reading a few posts. I suggest you just drop by your music store and see what's available. talk to a few sales guys at the Pro Audio/ Keyboards department and I'm sure you'll find something that works for you.

-ED-