Well, get a DS now. It plays GBA games, so you can get immediate use out of it.
Remember, the PSP is only due for a Japan-only release this Sunday, and most of them are already sold out. It will cross the Pacific well after the New Year, and probably in spring 2005. So between now and then, you've got plenty of time. (Plus, Sony is experiencing production shortages, so...). More a matter of what you can do now, and buy a PSP when they come down in price (DS is $150. PSP will easily be over $300). I would probably wait until Christmas 2005 before realistically wanting a PSP (hopefully the price would be around $200 by then).