I leased an '05, 4 years at 18k miles per year, and I've had it almost 2 years. I'm getting the itch to do some modifications, and love the car so much I don't see any need to turn it in.

I haven't talked to the dealer yet, but how expensive do you think it is to buy out the lease? Or am I better off waiting until the end, then buying another one (maybe a year old low mileage option)?

I would wait. When you go back to return it, they are going to beg you to keep it and will probably sell it to you for a great deal.

the buy out value is determined the day u sign a lease...i got mine in 05 and the buy out for me would be $19k with 45,000 miles in 2008...

as far mods go ive owned my G for a year look at my sig

To 'buy-out ' the lease, you would have to know a couple of things: Your monthly payment amount times the remaining months on your lease. To end your lease, the finance company is going to want you pay the total of the monthly payments that you agreed to at the lease inception. Once you pay that amount, then you have completed you lease obligation and would be free to walk away.
This is different than the residual value. That amount would be the amount to you to buy the car only after you have completed the term of your lease. The amount is spelled out in the lease documents.
Typically, the dealership has no part in the buy back of individual leases..it is an agreement between the individual and the finance company.
Hope this helps.