Butt Seriously folks...
OK, 'nuff fun here, dude asked a serious question... Sorry b-face, here goes -
What percentage of what you want to do is r&b, and what part would be rap? This will make some difference as to what you should add to your setup first.
If you plan on more rap, probably a better drum machine with more variety of sounds. With sonar, you could use virtual drum sounds as a VSTi instrument and save some $ -
If you plan on more r&b, a better keyboard should be more of a priority. Either the Motif or the Roland Fantom would give you a much smoother keyboard with more keys and fatter, smoother sounds for those huge r&b key pads.
In either case, you didn't mention a microphone. For rap, a Shure 57 or 58 is a good start. For r&b, a decent condenser mic with stand, shock mount, and pop filter would help the vocal end of things - there are decent ones for $200 or so. If you get a condenser mic, the next thing would be a separate preamp to bypass the mediocre ones in the inexpensive Behringer mixer.
In either case, you'll probably want to upgrade your monitor system to include a subwoofer, so you can hear lower harmonics and make sure they're not over the top on levels.
Figuring around $300 for drum software, $1800 or so for a quality keyboard, $300 for two mics, maybe $500 for a preamp, and at least $400 for a powered sub, you're at about $3300. You should be able to do better on a pretty usable preamp, so it looks like your budget can be maintained and still make noticeable improvements in your system.
Let us know whether your main interest is rap or r&b, and we can start narrowing things down more... Steve