Well depending on the OS, for the win9x, if i remember correctly upgrading from 64MB to 128MB will result in a theoretical increase of 8%, while around 10% for the NT based ones... (im not completely sure bout the numbers, i read it from somwhere). Anyway, having some spare RAM is always ok, but the type of the chip used also matters (i.e. CAS 2 vs CAS3), so you should consider that also.
The actual performance impact of upgrading RAM depends on what type of applications you are running. For example Office applications such as WORD, dont really need a lot of RAM. But simultaneously running Word,Excel,Powerpoint,Photoshop,Winamp,MSdev... etc while also say, compressing sumthing on the side, will definitely take their toll on the amount of RAM that you have and as well as the CPU type, mainly because of too much swapfile usage. There are things you could do to save memory, like limiting your disk cache (i.e. using a utility like cacheman), removing unnecessary startup programs, automatically unloading dll's from memory(i.e. cacheman?), stuff like that.
Finally, my suggestion is add 128MB/64MB to your system now(cost difference between the two is not much <$5), don't upgrade to celeron 800, it wont make much difference. Save you money, then upgrade at a later time, and get a cool rig.
The actual performance impact of upgrading RAM depends on what type of applications you are running. For example Office applications such as WORD, dont really need a lot of RAM. But simultaneously running Word,Excel,Powerpoint,Photoshop,Winamp,MSdev... etc while also say, compressing sumthing on the side, will definitely take their toll on the amount of RAM that you have and as well as the CPU type, mainly because of too much swapfile usage. There are things you could do to save memory, like limiting your disk cache (i.e. using a utility like cacheman), removing unnecessary startup programs, automatically unloading dll's from memory(i.e. cacheman?), stuff like that.
Finally, my suggestion is add 128MB/64MB to your system now(cost difference between the two is not much <$5), don't upgrade to celeron 800, it wont make much difference. Save you money, then upgrade at a later time, and get a cool rig.