Contrary to what Colyn says, if you choose your body carefully, you get full functionality that the lens offers. My Nikon D80 works perfectly with lenses from the 1970s! For the most part, it comes down to how the autofocus is driven (if the lens has an internal motor, or if it is driven by the body). If you buy a body that has this built-in motor, you can use all the lenses. The Nikon D40/D40x, for example, don't have this.
But, I too would strongly urge you to rather buy a Nikon or Canon. When it comes to DSLRs, your biggest expense is the glass, not the body. You'll probably end up replacing your body in a few years' time, but you'll keep your glass for much longer.
I don't know much about the 75-300mm that you own, but Nikon have 70-300/5.6 lenses starting at $120! I bought one and it's crap, btw, but the point is that you could potentially replace your existing investment relatively cheaply unless it's very expensive glass. The flash is another matter, but I got my SB600 for ~R1200 from Amazon (you'll need a friend in the USA!) And it needs to be said that flash functionality has come a long way since the early days of SLRs - this thing is wireless!
Nikon and Canon are the big players and if you're going to be investing in SLRs, you're better-off going with one of them. This way, you won't end up in your current position again (having glass and no body for it!)
Not only are the Canon/Nikon ranges bigger, but the glass and accessories are cheaper and more readily available. As you probably know, the body is only a small portion of the total cost of ownership of an SLR.
Before buying a camera, I would suggest you go'n play with them in the store. When I bought my camera a year ago, I almost got the Canon Rebel XTi (400D), because it was much cheaper and is technically much the same thing as the Nikon D80. However, a friend suggested I try them first and after playing with them, I found the Nikon much more intuitive and the controls easier to use. The Nikon was also a little bigger and heavier and it felt "right" to me. At the end of the day, you don't want to be fiddling with the camera to get it to do what you want. You should be able to change things quickly without having to dig into menus to get the shot you want, before it passes-you-by ! They should be natural to use. That said, many of my friends have the 350D and 400D and say they got used to the controls. So maybe you can learn 'em. I don't want this to turn into a Canon vs Nikon thread!
Both Canon and Nikon now have new ranges out, and I am not intimately familiar with either, so I can't make a recommendation to you. But dpreview is a great resource. Use it to narrow your options down and then go'n play with your shortlist in a store to make the final decision.