I always recommend that you have a spare card inyourcamera bag and that the primary card or cards have enough space to capture the number of images you are going to take over a period of time that you can not off load the memory card. Thespare card should be in addition to the number of images you plan to take and be there in case something goes wrong with the card or one of the cards you use.
My camera is a 10MP camera and I keep a 16GB in the camera but also carry an 8GB and a 4GB in the camera bag.I also own a 2GB cardthat I have in my spare camera.I never plan or have had a situation where I would need more than the 16GB and the 8GB cards so the 4GB is a backup card for me. The combo of the 8 and 16GB has gotten me through an entire day of wedding shooting and is more than I have ever needed for the largest newspaper and fire-rescue events I have ever been involved in.
What I would do is buy the faster card you can afford, although you camera may not be fast enough to take advantage of it as many people have pointed out on this forum is that a faster card can upload to the computer faster and after taking 100s of images that can save you a lot of time. I would look at ther class 6 SDHC cards if you can afford them.
Also like Tcav sayid we don't have to pay for film processing anymore so why not spend a little extra on the memory card because you can reuse it for years into the future., I still have a working smart media card from 2002, but the camera is long gone.