Though you meet the minimum required specs the recommended specs are 1.8 GHz or faster Intel Pentium IV or AMD Athlon processor 256 MB or more RAM
1.8 GB free hard disk space plus space for saved games Nvidia GeForce3™ or more recent Direct3D™ capable video card.
I myself played this game (I really like it) on a PC with an Athlon 64 3200+ cpu (2.0 GHz), 2 gigs of RAM, and an X800GTO 256mb DDR3 video card. It
more than exceeded the recommended requirements but even then it would slow down if a lot of units were on the screen.
One rule to keep in mind is to have 2 gigs of RAM if you are gaming on an XP system. RAM is cheap today (older can be more expensive) but you'll need to know how much memory your motherboard can handle, plus what kind it takes. Go to
http://www.crucial.com/systemscanner/ and use their free scan. This is completely safe and very reliable. It will tell you how much RAM you can add and the type. Then if you desire to purchase go to Newegg.com and buy the memory you need for less $.
The other thing you would need is a new video card but it appears either your card is a PCI or AGP and both of those types are basically no longer manufactured. Today it is PCI-Express and your motherboard in all likelihood isn't designed for these video cards.
However, you can buy both AGP and PCI cards at Newegg.com for a decent prices and a number of them will be much more powerful than your current one.
This is what we need from you: Your system specs such as your make and model of your PC (if bought from HP, Dell, or etc.).
You said you updated everything. Does that include your video card driver?
If you don't mind me saying, my recommendation is to begin saving up your cash for a new system and have someone build you one. It will save you money in the long run and you can build it as you want.