Wedding with a D90: How should I enhance my current set up on a budget?

I've been asked to shoot a wedding for two very good friends of mine this coming June. Right now, I'm running a Nikon d90 with the kit 18-105, a 50mm 1.8, and the 18-55 and 55-200 from my d60 kit that I started with. I also have the SB-600 flash unit, and I still have my d60 as a back up body in case of emergency.
It will be my first time shooting a wedding, although I do have experience with event photography (I have my diploma in journalism and spent a full year working for my college's newspaper.)
The wedding itself is outdoors, but the reception hall they have chosen does have fairly low lighting.
I will be able to save up about a thousand dollars for new equipment before the start of June, which would give me about 3 weeks to practice before shooting the wedding itself. My question is what the best equipment for me to buy would be, knowing my current set up and experience. I've been looking at picking up the SIGMA 18-50MM DC F2.8 EX NIKON HSM MACRO, for starters, and maybe a second flash and extra battery. Is this the best route for me to go?

Added (1). I should add some extra details to this after my first couple answers. I do in fact have a 5ft extension for my flash already, as well as a diffuser (but not a very good one - I was planning on picking up a fing anyway.) I have a second shooter as well, she runs with a d80 but has no extras except for a 50mm prime.

Also, I'm sure most people are going to point out (and trust me, I already know this) that it would be better for my friends to hire an outside, professional photographer. This is not even remotely a budgetary possibility for them (we've looked, photographers in the price range they can afford in my area are ghastly at best), and they are aware of and comfortable with the fact that this is my first wedding shoot.

With the Sigma lens I could afford to buy an SB 900. With the Nikkor version, I'm going to be lucky if I can buy a battery, unfortunately.

Some (somewhat) recent shots of mine are available at my flickr account, if see

Added (2). Oh! I'm also completely willing to sell my 55-200 to add to my budget, that thing is a useless hunk of junk anyways.

Plenty of batteries all around and do consider at least renting a backup flash. Also make sure you spent some effort on expectation management, you want to keep these people as friends.

While we're on the subject of renting, why not get a 24-70 f2.8 for a weekend and see what you think? That lens is so awesome that a competent photographer with little information with that specific lens can still make it perform.

Even so, you're kididng yourself if you think you can shoot it all with available light, a good wedding photographer needs to be a flash master, both indoor and outdoor.

You have got enough lenses at present, but certainly a constant f2.8 standard zoom would help. The SB600 may be slightly underpowered if you are shooting big groups in a dark hall. You must get some means of holding the flash off the camera, even a one-metre extension would help. Get a second flash that can be radio-controlled if you are taking an assistant along with you. Get or make some sort of diffuser for the flash, practice using bounce flash. You will certainly need plenty of battery power.

If this is your first, you are going to find the stress levels through the roof…

ask them to use a professional, then you can work around in the periphery and take things at a more leisurely pace, until you are happier with what you are doing.

you wouldn't want to ruin someone's "special" day…

If you can't afford the Nikon 17-55 f/2.8, the Tamron 17-50 f/2.8 is a decent substitute. The 24-70 is a great lens, of course, but won't be WA enough on the D90. That would leave enough left over for the flash and an extra battery. Your Flickr is competent.