It really depends on whether you want to do any sort of mobile computing or not. If you do, get the MacBook. If you don't, get the iMac.
Also, I wouldn't follow LarsG's advice about waiting for a possible upgrade. The fact is, you will always get a faster computer if you wait a few months. Waiting for it to happen is a false economy, because you have lost out on having the new computer for those few months, and end up in the same situation again, where if you keep waiting it will just get faster.
Once August rolls around, you could just wait until October to get Leopard. Of course, if you're willing to wait until October, you might as well wait until the pre-Christmas sales push to get another little bump in processor speed. Etc, etc. If you keep waiting long enough, maybe you can get your grandchildren to buy it for you, and you won't have to pay a dime.
Also, I wouldn't follow LarsG's advice about waiting for a possible upgrade. The fact is, you will always get a faster computer if you wait a few months. Waiting for it to happen is a false economy, because you have lost out on having the new computer for those few months, and end up in the same situation again, where if you keep waiting it will just get faster.
Once August rolls around, you could just wait until October to get Leopard. Of course, if you're willing to wait until October, you might as well wait until the pre-Christmas sales push to get another little bump in processor speed. Etc, etc. If you keep waiting long enough, maybe you can get your grandchildren to buy it for you, and you won't have to pay a dime.