It's more of a question about what you want. If you want something more expensive, you'll have to save up longer. If you're not a big or extremely tall person, then the 250 or 500 Ninja are great starting bikes.
Now, myself, I have a '2002 250R Ninja and love it. Being as I weigh a mere 120lbs, this thing has no acceleration problems. The 250 Ninja isn't known for blazzing top-end speeds, even though I recently took mine out with a GPS and took a speed fix at 109MPH - with engine but no road to spare. The 250 Ninja is widely known to be the king of twisties.
Have you also thought about insurance rates? How old are you? If you're under 25 yrs/old, check with your insurance agency before you buy a bike. They may or may not cover you; a lot of insurance companies will not insure a motorcycle, period. Also, be prepared to fork over insane amounts of $$ for routine maintenance at your dealer. I'm not saying that all dealers will rip you off, but most will try. If you're a handy person, try to do most of the maintenence yourself and save $300-500 a year. Dealers around here and up East are known to charge $150 for the 300mi checkup on the Ninja. That checkup includes things such as changing the oil, bleeding & checking the breaks, making sure all bolts are fastened correctly, checking/adjusting valve clearance, ex.. If you have the tools, anyone can perform these tasks without a problem.