As others said, it all depends on the media/apps you plan on having.
The phone has 8gb internal memory (about 6.5 available upon shipment). The 2gb card is alright to start off with (some apps require a memory card, thus why its supplied), but won't go very far. As of now, the phone can take a 16gb card, but will be able to supposedly take a 32gb card with Froyo. And with Froyo, you'll be able to save apps to the memory card. AND we might have 720p video with Froyo as well, which will also take up more time.
But, if you don't plan on having a ton of music, pics, or videos on the phone at one time, you should be perfectly fine with a 4gb or 8gb card.