The 500D is a nice camera, but Canon's crop cameras do lag behind the FF cameras. You are probably making the right decision to change cameras and a 5Dii is a smart option for many. But some 1Ds and 1D models are in your price range and they have some positives, too.Even ignoring the image quality, I found just the AF-ON button, the custom-modes, shortest shutter speed, second wheel, top LCD, the view finder etc. a huge plus.
But personally, it wasn't really until I got a FF camera that I realised how immaterial the whole APS-C vs FF debate is. I'm now using a little APS-C Fuji for much of my photography. It might be technically inferior to my Canon, but the difference isn't that big and it is better suited to what I do.
So the only real suggestion I have is to sit back, evaluate your photographic goals, work out where your 500D is deficient and then decide which camera is best suited for your needs.
Of course you don't need to go full frame for all of these features, but an older 1-series body as you suggest would have them, too.