You can add all the RAM you want to it, but if the OS is 32bit (so not 64bit), it can only use ~3.5GB of it.
Also mixing RAM is generally not a good idea. If you are going to put in a new kit, remove the existing kit beforehand.
If you have a 64bit OS, put in as much RAM as you need/want.
If you don't, put in 4GB of RAM, anymore would be a waste.
Just realized your mobo only supports DDR2 RAM.
Heres a 4GB kit of DDR2 RAM.
G.Skill 4GB 800Mhz DDR2. $50
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231207
Though replacing the PC outright at this stage wouldnt be a bad option. As Idroid has pointed out already, the system is quite old. It relies on defunct standards/interfaces like DDR2 and IDE, so the upgrade path has essentially none.