It doesn't sound like you need to upgrade. As a 3G user, my situation with my phone does call for an upgrade. The 3GS supports multitasking so you should be good for a year. And next june they will release another one, which will be better than this one.
Im in the same boat as I too have an 3G and have been waiting to upgrade. Though the 3GS is a good improvement on the 3G, IPHONE 4 is a huge jump from the 3G. So since Im going to obvisiously upgrade into a phone. The multitaskiing, folders, nike plus support, better form factor, facetime, display, hd recording, gyroscope, etc, etc. has me salivating. I cant wait to upgrade, but if you dont really need to jump in the IPHONE 4, dont.