Whatever you think is a good buy. It's been shown that the cheap value ram is actually pretty good, especially when measured by performance/price ratio. Any decent name brand ram that is pc3200 would be fine. Your 4600 can take advantage of dual-channel ram, so perhaps a set of 2x512 MB sticks of pc3200 DDR dual-channel ram would be best. Corsair makes good value ram, and I'm sure you can still buy a set of 2x512 for ~$75. May I ask why you need more ram?
I am a little confused at the moment. What are the specs of your current Dell Dimension 4600 (how much ram currently, what Intel cpu)? As far as the possibilty of re-using these parts in a future build, the 4600 will only allow you to re-use your ram if you go to socket 939 or 754 AMD, which are themselves becoming obsolete (everything is moving to DDR2). The only things that would be considered usable in a modern computer build in the future would be the hard drive and the optical drives, unfortunately.