By rgb cable I'm assuming you mean component cable?
I got by just fine without HDMI for quite some time. I still use component from my cable box to my TV. It gives, arguably, as good a picture as HDMI.
The biggest advantage, IMO, is HDMI's ability to carry the HD audio signals from a BD player to a HT receiver. At this point HD audio is not available thru cable or satellite.
Somewhat off the subject; The picture quality will depend on the scaler in the TV you buy. Some SD broadcasts look horrific on my RP CRT, sports in particular.
I do agree with some that it would be a waste of money to invest in a HDTV without the investment of an HD source. But that investment doesn't need to be a BD player. Check out your cable company's HD offerings. I still find the some of the HD broadcasts to be amazing.