Relax, this is very simple - really! ALL YOU HAVE TO IS MAKE SURE THAT MEASUREMENT "L" IS MORE THAN 3 INCHES LESS THAN YOUR INSIDE LEG, BUT OTHER THAN THAT CHOOSE THE LARGEST FRAME! So if the 550 gives you 1'', the 530 gives you 3.5'' and the 480 gives you 6'', go for the 530.
You can then adjust length for your arms by fitting a new stem - or flipping the current one. But you probably won't want or need too.
If you're fairly heavy, make sure you have at 38mm tyres on for the type of riding you're doing. Ergon grips and bar ends are also worth considering, and I'd always pay to upgrade a mid-price bikes tyres to something like Conti Contacts or Marathon Plus. Or for the ultimate in comfort fit Schwalbe Big Apples - the Nike Jordans of tyres - if the bike will take tyres that wide. Swap the brake pads to Kool Stop Salmons if you can, especially if you will ride in the rain. Don't choose a bike on how much you like the saddle either - saddles are easy to change and you should get a refund on the swapped out one if you do this when you buy. A WTB Speed V is a good bet for a heavy rider.
As for the incompatibility of the sizing systems, it's a function of the exact frame geometry. Ignore it. It's that crucial "L" that matters. Remember that you were at the size borderline for the Roadrat anyway.
Last edited by meanwhile; 08-29-09 at 06:00 AM.