pSI Civic
I get the Integra up to about 196mph on the banking in 5th gear, then I hit both 6th and the full-strength nitrous just as I enter the main straight. The trick is getting it up to 196mph in non-nitrous mode. The rest is easy.
Basic method is to tune the car to go fast without using nitrous. Once you think you've got it going best, figure out the 6th gear figure by multiplying the 6th ratio by the final drive ratio (for example, 0.574 x 4.640 = 2.663). Now set up 5th gear so it calculates to be just a bit shorter than 2.663 (eg, 2.720 or something like that, using the above example). You may have to alter the final drive number to get a good result. However, doing that will get your car up fairly close to its maximum non-nitrous speed by the time you hit the nitrous (and 6th) at the beginning of the final straight. Then all you need to do is figure out which gearing for 6th gets you to the end of the straight at the highest possible speed.
I'll have to give the Xsara a run, seeing as how Casio got a good result out of it.