How much do you think my MBP is worth?

I just read about Liteware's education promo and I've been eyeing the new 13inch MBP ever since it was released, so of course I'd like to upgrade... I'm asking how much my current mac is worth coz if it costs a little more than the new mac I plan on selling it. anyway, here are the specs:

Macbook Pro 15"
Model Name: MacBook Pro
Model Identifier: MacBookPro3,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.2 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 4 GB
Bus Speed: 800 MHz
Chipset Model: GeForce 8600M GT
Type: Display
Bus: PCIe
PCIe Lane Width: x16
VRAM (Total): 128 MB
Vendor: NVIDIA (0x10de)
Device ID: 0x0407
Revision ID: 0x00a1
ROM Revision: 3175

hard drive: 500gb
battery cycles: 86

-bought last jan 2008 in the US... so no box. But I have the install discs, sticker, remote, charger, display adaptor, everything
-just had the hard drive and ram upgraded last april/may (thanks to Sir Charlie)
-there's a sand grain sized ding?/really small indentation about a cm to the left of the touch pad. it's super shallow and is only noticeable if you're looking at it up close. I'd take a picture but on my 2 megapixel cellphone cam it's not visible.
-no applecare. never had problems with it though

I hope someone can help me out here. I checked the marketplace and wasn't able to find a FS thread about an MBP with specs like mine and similar models were sold but the threads are from last year. Thanks in advance

 

Thanks a lot! But I actually decided to just keep my mac, the great thing is even if it is technically outdated it doesn't feel like it at all. Great machine Just curious, does Apple still produce/sell batteries for my model? I'm sure I'll have to replace it eventually and I hope it's easy to get.