I would upgrade or if you can, wait to see if they will be having a 3DS deal soon, the system isn't selling so there may be a bundle, get a free game or a price drop offer soon. I would expect there to be some type of deals regardless when Zelda comes out in June.
If you don't upgrade, I wouldn't pay more than $60 for a used DS Lite, ideally a lot less, more like $30-40. Eventually Nintendo will discontinue making DS games and although the DS has a huge huge library if you want to play current stuff especially when it comes out you should upgrade. Its kinda pointless to invest in a DSi or DS Lite now as the 3DS is backwards compatible with those system. You could have skipped the whole DS line and still get everything with the 3DS. But I personally would not have wanted to do that as I enjoyed the DS line of systems very much.
If you are a fan of Pokemon note that Pokemon games like Diamond, Pearl, Platinum and HG/SS, if you upgrade to 3DS or DSi you won't be able to trade in Pokemon from your GBA games, and there are a couple more games that won't work without a GBA Slot like guitar hero. For some people this might be a dealbreaker so I am just mentioning it here.