I wouldnt even bother with 3Gb of ram unless your going to install 64bit Windows XP or Vista.
Sure 32bit windows can use PAE to access 3GB, but it will still only assign a maximum of 2Gb to any single application (With the exception of Microsoft SQL server which has native PAE support). Thats a limit of 32bit windows. So unless your runing many programs at once, and using up all your ram, an extra gb wont do much. Even memory hungry applications like Photoshop are not compiled with PAE support, so can only use 2Gb ram. (Of course it does mean windows can allocate 2GB ram for photoshop for example, while the OS itself is using other parts of the memory... hence as I said its ok for heavy multi tasking)
That said, as you only have 1Gb of ram.. The move from 1Gb to 2Gb is very worthwhile. Im into MMORPG's and frequently my system is using around 1.5Gb of memory.
PS, DDR2 isnt even 'always' faster. AMD64, and P4 both prefer low latency to high bandwidth(As long as this isnt taken to extreme... IE even if the imposible was available (zero latency), if combined with extremely poor bandwidth overall performance would be bad. Its one of those balancing acts Having the highest bandwidth without sacrificing too much latency is often the way to the best performance machine. DDR memorys often have extremely tight memory timings (as fast as 2/2/2/5 is quite common on ddr). While DDR2's are more common at 5/5/5/15, although the latest DDR2's are getting better, 3/3/3/12 is pretty respectable