Right now the Xbox 360 base unit is trading in for $100 store credit. That's without hard drive and with one controller. GameStop trades in the hard drives separately as they have to send them off to be cleaned. The smallest size, 20GB, trades in for $30, with the 60GB, 120GB, and 250GB netting you $35, $40, and $50 respectively. Then there's the network adapter. The new system has built-in Wi-Fi, so your wireless adapter is now obsolete. The white version trades in for $40, while the newer black version will score you $50 credit.
And let's not forget the Edge card, which gives you an extra 10 percent store credit for accessories. So, if you've got a 250GB Xbox 360 with the black wireless adapter, your trade-in credit works out like this:

System: $100
Network Card: $50 ($55 with Edge card)
Hard Drive: $50 ($55 with Edge card)
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Total: $200 ($210 with Edge)

With the new Xbox 360 selling for $299, that leaves only $90 to $100 unaccounted for. Other items you can trade-in include wireless controllers at $25 apiece, and wired controllers for $20. So yes, trading up to the new 360 isn't as painful as it could be, and GameStop told me that if they sell out, they'll let you reserve from the next batch, with the trade-in special still applying. Of course you have your saved data to worry about, but that just means you'll have to play your favorite games all over again, and who doesn't love doing that?

Update: Someone named Fred informed us that GameStop will give you the trade-in price for your hard drive up to seven days after your purchase, meaning you can transfer your data and bring the drive back later and still receive the credit.