A new needle for B&O RX2 turntable or get a new TT

For that kind of money, you could get a used Rega P3 with arm and an inexpensive cartridge, and it would probably be better than the B&O. Those proprietary B&O carts are too much money for the sound you get.

Since the B&O is designed as a system, there should be good synergy between all the parts. If you go the new TT route, try to listen to the entire thing with the cartridge you'll be getting, as randomly throwing together a table, arm, and cartridge may not be an improvement. It is important to match the components together, but I have no idea how. Years ago, I bought a Dual 616Q table, not highend, but widely respected. I did listen to a B&O right next to it (don't remember model), and it was significantly better, but more $$. I went back in a year to upgrade, they talked me into a Linn Axis & Tonearm. I ended up putting cartridge from Dual on Linn, brought it all home, it didn't sound much different than the tapes I had made from my Dual. They said it was a lack of synergy between the arm and cartridge, I should get Linn cart. I ended up taking back my Dual, & would have been better off getting B&O.

I have an old RX2 with the same problem; the MMC4 that it came with is discontinued. I bought the MMC5 and for my money and time spent with a turntable, it has been a good value. Vinyl has lower on my priority list, however, so I would say it depends how much you will use it. Another brand makes some sense because you may use any brand (or price range) of cartridge and may also upgrade parts, as in the case of the Rega. I have been hesitant to upgrade my B&O as I am relatively happy with it. I wouldn't invest the big bucks in a high-dollar cartridge as the B&O dealer suggests, as the turntable is worth less than the cartridge and you will never be able to sell it for close to your investment should you decide to get rid of it.

I used to have the B&O Beogram 3000, which used the same type of cartridge. (I used the MMC 3, btw). Here are my thoughts on the matter:

PRO:
B&O turntables are at the peak of the mid-fi turntables.
They are easy to use, tweak free, and produce a nice sound.
Also the cartridges they use, while a bit on the expensive side, are light enough and small enough, that they will not ruin your record collection, assuming you do a decent job of cleaning both your records and your stylus.
(I know this, because I used mine for about 10 years or so, then put away my table for CDs and did not get back to vinyl for another 15 years or so. The old vinyl still sounds great through my new table.)

CON:
B&O turntables are not really in the high end class, IMHO.
They do not image or soundstage well enough, and their frequency response is somewhat limited.
(I found out what a true turntable could do when I upgraded to a Basis 1400, with Rega RB300 arm and a Benz Micro Glider II cartridge.)

My opinion is that you need to make up your mind as to how much you want out of your turntable. If you are making CDs your primary source, and only want to play a few records now and then, it probably makes sense to pick up a MMC 2 (or MMC 3) cartridge. (I think the MMC 1 is way too expensive for what you are getting.)

If on the other hand, you want to really make the plunge into vinyl, then sell the B&O and get a used Rega P3, or a something similar. (A used VPI scout would probably be even better.) Of course this would mean spending twice what a MMC 2 cartridge would cost.

Good Luck!