On a 32-bit version of Windows you won't be able to access all of the 4GByte of RAM. If you want a general speedup, use an SSD. According to TLom's hardware, you hardly go wrong with the brands Intel and Samsung, but any brand will give your computer a big boost.
You might even want to consider doing a fresh installation of a 64-bit Windows version. All the "speediness" from a fresh install with 4GBYTE of RAM and an SSD..., you will be amazed about the leap your old computer is able to make.
Then again, the money you will spend on getting your old system fast will also let you buy the core of a new PC (motherboard, CPU, RAM). For example, my Asus motherboard costed me about 100 USD at the time, and decent boards are still had for that price. My CPU was around 200 USD and nowadays 200-250 USD will buy you a decent i5 and spend 50 USD more for 8GByte of RAM (2 modules of 4GByte is better+faster than one module of 8GByte).
You can still use your old PC case, the old power supply is usually capable enough, a PCI express video card can be re-used and your SATA/SATA2 hard disk will work good enough as well, especially with a fresh Windows installation. An optical disk is hardly necessary anymore nowadays, so if you have one that still uses IDE, you might as well drop it. Most new motherboards do not come with physical IDE connectors anymore.
So if you are handy and able to do the work of swapping out motherboards etc. yourself, you'll spend around 400 USD to get a new PC. That amount of money you will also spend on DDR2 modules and an SSD for your old system (prices are just an indication and are based on prices in Paraguay, US prices are normally a bit lower).
With the new PC you can later on spend around 100 USD more to get a new SATA3 disk (1TByte) and a somewhat decent new 1GByte video card. After that, spend 20 USD more to get an el cheapo PC case (with power supply) for your old ECS motherboard and restore that old system with the old hard disk and video card. One (recommended) use is to try Linux on it, you could use it as an extra/backup PC, sell it for whatever pittance you still can get or even donate it to family/poor/church/whatever.
Decisions, decisions, decisions