I think you should go with a DS Lite, an acekard 2i and an EZ flash 3-in-1. That should give you pretty good compatibility and the ability to play the newer DS games along with your old GBA games. Or you could wait until the DSTWO or iEvo are capable of near-perfect emulation(could be possible if DSi mode is cracked) and get an XL.
EDIT: Also, you should post in the Ask GBAtemp section, so others who have the same question won't have to make another topic.