Well in my opinion SP3 SHOULD be better, and as MagicAndre1981 said "A Sp is a collection of updates and fixes" and this things should help, but if your to lazy it isnt quite necessary.
Good guide for the ones with lower memory, but I dont suggest Vista on 256 MB of RAM, neither I would suggest it on any desktop / laptop as there is XP that is better for the ones with low memory and Windows 7 that is like a better Vista.