Your set up is pretty well matched, any one change will make a small difference, but not a huge one until all three are switched out. My opinion here, but I'd upgrade the turntable first, keeping mind the level of new cartridge you are aiming for, then the preamp. Normally I'd suggest the cartridge first, but I own a Raga P6 and Rega's are known for not being very adjustable, so you might end up with a cart that is not that much of an upgrade. Therefore, the turntable first, so you can upgrade to a much better cartridge, then the pre amp to get the most out of the new cart. That's what I would logically do if I couldn't do the whole thing at once..
