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The easiest upgrade & the most beneficial one, will be the ram. 128MB is amazingly little memory for a system of that spec. I'm not surprised its running a little sluggish!
You have to be careful when you go to buy extra memory for any system, Dells in particular. Not only must the memory be 100% compatible with the system, but it must also be compatible with the existing ram as well, otherwise you'll run into problems.
Unfortunately, simply buying a new stick of ram that appears to match the specs of the existing stick, is no guarantee that they will be 100% compatible together.
There is a simple solution though (apart from buying it from Dell who will charge a lot). Go to Crucials website (www.crucial.com), select your country & then use their memory advisor. You can either opt to have them scan your system or you can select the make & model (i.e Dell Dimension 2400) from the drop down lists.
Crucial will then give you a list a memory modules that they guarantee to be 100% compatible with your system. I strongly suggest you buy your memory from them, as opposed to trying to find a stick that matches one in Crucial's list else where. Crucial are not expensive, they guarantee compatibility & their memory is very high quality.
Upgrading the RAM will give you a more significant boost in performance then any other single upgrade. You could probably upgrade the cpu a bit but there will be a limit in terms of the fastest cpu that your system will support. You will probably find that you can only upgrade to one a couple hundred MHz faster than your current one & this won't increase the performance significantly.
If you play a lot of games or use graphics/video editing programs a lot, then you may wish to upgrade the video card as well. However you will need to find out first whether your current card is integrated or not. If it is, you will need to have an agp slot on the motherboard as well in order to add a new card.