Should I purchase an expensive phono preamp

You can easily improve on the MM phonostage that's in the receiver, and for not a lot of money -- certainly not what the stages you mention will cost. You have a nice TT and arm, but your cartridge and receiver (more the receiver) are the limiting factors sonically, and so a very modest phonostage will get you all that the cartridge and receiver are capable of delivering IMO.

One day when you have the money, you could invest in a receiver from one of the better domestic manufacturers. It will most likely have a great MM/MC phonostage built in. Save your money for now.

Well, I'd keep it under $200. That lets out even the Creek OBH-18 by $25. But there's the Parasound or Project Phono Box for $150/120 from Music Direct, or from Audio Advisor the little Bellari Rolls tube unit for $150 -- also doubles as a headphone amp, and will add the warmth of tubes to your (very very solid state) receiver.

Ejcpa, what cable are you (will you be) using from your tonearm to your phono preamp?

The Origin Live tonarm cable is an (maybe THE? I don't know) upgrade for the cable that came with. See:

http://cls.audiogon.com/cgi-bin/cls.pl?anlgtnrm&1145122254

But maybe this isn't the time (for you economically) to start upgrading those items. From the Bellari to the receiver, just make sure you use a good shotgun type, low capacitance (cause of the MM cart,) shielded cable. You can get Straightwire Maestro I or II, Audioquest. MIT, Purist Musaeus, all bargains new and especially used. I'd stay away from Cardas (too warm added to the Bellari) Kimber or Nordost. IMO

If the tonearm cables are hardwired to your Rega, I'd just let it be for now. Whatever they are. the'll be good enough for what you're about to put together.

Later on, you might decide to get an altogether new arm for your TT, new arm and TT, or maybe upgrade the Rega at britaudio as Dan_ed suggests. For now, just get a decent entry model pair of ICs (Bellari to receiver) from one of the above reputable manufacturers and you should be fine.

EJ, yes, they all have very low capacitance specs. And it's not critical for the phono preamp to receiver connection.

The critical part for MM carts. is from the tonearm to phono preamp connection. The Goldring 1000 series specified capacitance is 150-200 pf (picofarads). To achieve this, you'd have to know the capacitance of the Rega tonearm cables and add that to the default capacitance in the Bellari and then either add or subtract capacitance to meet that spec. by using little load capacitors inside the Bellari. I don't know (doubt) if the Bellari provides for that amount of tweaking.

Bottom line, don't sweat it for now.