To riff on John's recommendation a bit - Roland have a new synth hardware module out that got pretty high marks from Keyboard magazine - basically all of their synth engines in one box - but limited to 128 note polyphony. It's called the Integra 7. http://www.rolandus.com/products/details/1245/462 Price is a steep 2k$ street.

You should be able to get latency down to under 5 or 6 mS, which would be very hard to detect, however perhaps not with the laptop built-in sound card.