Since you have a DS now you might want to go with a DS flash card and an EZ 3-in-1 that can function as a GBA cart (In addition to a rumble pack and RAM expansion for some homebrew games). I bought my EZ-Flash V from DealExtreme:
http://www.dealextreme.com/details.dx/sku.1893Nice package, and the card's been getting lots of updates that are making it competitive with all the competition. It's considered lower quality by a lot of people but I don't really see why, I don't use it for running all that many commercial games but everything I've tried on it has worked flawlessly. If you get one of these I can easy help you get it up to date too.
Actual GBA flash carts are getting harder to find and they're a lot more expensive than the DS cards with a lot less capacity--the 3-in-1 may require you to reflash every time you want to switch games (Actually, not anymore, another firmware update allows for multiple ROMs on it at once as long as the total is less than 32MB) but you can fit a ton of games on even a 1GB microSD card.
If you order from DealExtreme be prepared to wait a
long time for your order though, it took a few weeks to get here but free shipping can't be beat, especially when you consider what some of the other sites charge you for the same shipping method.
This is probably a better deal, it's the new model that supports SDHC cards:
http://www.dealextreme.com/details.dx/sku.13681Doesn't come with the memory card but if you've already got one that's not a big deal, and you can buy them just about anywhere nowadays. You might want to wait a little longer though, there's also a new 3-in-1 Plus that's supposed to come out with more storage space for GBA ROMs.
