bring your bag, but take it off to race. Computer is pretty unecessary. One large water bottle is probably enough, but realize you'll be going hard so take more water than you would for a similar length training ride. As for warm up depends on the type of race and the category. Cat 5 crits usually accelerate to maximum efforts from the gun, so you need to have done a fe short maximal efforts in warm up. Road races, particularly in higher categories often start out slower and you can warm up a bit in the race. I'd plan a little more than a half hour. Try 15 minutes spin, 5 minutues tempo. and then a couple of 2 minutes at 8/10ths, and 2 or 3 all out efforts for 30 seconds. Then 5 minutes cool down spin and then go race.