I agree with everyone about going AMD 64-bit. However, if I was going to make that jump I would NOT get the Sempron for your needs/wants of being able to upgrade later. I'm not saying it's not a good processor, but everyone keeps saying you can upgrade the video/cpu/etc later on.
This is only half true. The Sempron is socket 754, which is already being abandoned, so you'll run into the same problem that is being mentioned about no processors being available. This is going to be the possible case for anything you buy now. In 2 to 3 years, don't expect the CPU socket to be around anymore.
That being said, I would still go for the Sempron 64, unless you want to be able to upgrade the processor later. If you want the upgrade option, I would suggest saving up a little more and jumping up to socket 939. It is at least more likely to still be actively around by your time frame.
Either way I would stick to going AMD 64-bit. You will be glad you did.
