Usually it's the best time to do an upgrade, as you will get the most trade for your old system then you pretty much will ever again.
The problem is, they will only do one per item... I just happened to pick up a super cheap 360, and I called and asked about it. Each bonus has to be on a seperate transaction, and they can only boost on the same transaction as the sale.
I got away with it once, on a DS. I traded two GBA in for the DS, and got the bonus on both... Just don't count on it happening, unless your local shop is pretty lax