It can only support up to 2GB of ram and your upgrade path isn't good. You are better custom building a new and better PC which is very good and you will be recommended to do so by the majority of members here, Have a look at the DIY kits that Newegg offer Newegg.com - DIY PC Combos, DIY Computer Bundles which are very cheap.
For a computer this old, i'll recommend switching to an OS that is designed to run on older computers.
Checkout Linux Lubuntu. It will do wonders on that old PC compared to a windows OS
According to crucial scanner it allow's up to 2GB Computer memory upgrades for Dell Dimension 3000 Desktop/PC from Crucial.com but the OP will need to the run scanner to confirm if he/she chooses to upgrade the ram. Like BikerEcho said Lubuntu is a good idea though I've never used it I've heard good things about it.