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.

Any that you would recommend?

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.

You might want to look into modifying your RB250. Origin Live modifications to the arm make this piece a category killer that will improve sound defination, character, etc... . There are good sights on the internet where you can read about this. Also look into rewiring for the arm with something like Cardas. Check it out.