Should I get a new cart, TT or phono stage?

At some point i want to replace all three but I'm wondering what will make the biggest improvement first.

I have an old Denon DP-45F with a recently purchased Ortofon 2M Red going into a Pro-Ject Phono Box II which then goes into a Simaudio i1 integrated.

What would you upgrade first. Some things i was thinking of were a Rega P3, Clearaudio Emotion, Simaudio LP3 or used LP 5.3, or a ortofon 2M Black.

If I made any upgrade i would likely be stuck with the remaining components for some time before I made the next upgrade.
I would replace the Phono stage first
this unit is the most important in analog Playback
06-07-09: Djn04
One thought I had was to replace the stylus on the 2m red. I read somewhere that all 2m have the same body. Is that true? If so could I put a 2m black stylus into my 2m red body?
Although the four cartridges appear physically identical, they 2M system is really two separate cartridge bodies and four stylus assemblies. The Red and Blue cartridge bodies are identical. The Bronze and Black cartridge bodies are also identical--to each other, but not to the Red/Blue bodies. The Bronze/Black bodies are built with better materials and closer tolerances (and slightly different design IIRC) to take full advantage of the higher-res--but slightly different compliance--Bronze and Black styllii.

You *may* be able to slide a Black stylus onto your "Red" cartridge body, but it won't make it a real 2M Black. However, if you replace your Red stylus with a Blue one, it will be identical with the factory 2M Blue.
Some great advice here from everyone.
Phono stage would make the most noticeable improvement,
however, sounds as if your set-up is well matched at the present.
I would wait awhile before changing things up and enjoy.
Maybe spend that upgrade money on some new vinyl.
So I've taken the advice of many here and bought a Era Gold here on A'gon. I'll report back when I get it and I can A/B with the pro-ject.