How much Ram can i use

Hello,
I got this computer along with monitor, keyboard, etc... Turn it on and come to find out its kinda old which is no big deal was going to give it to my 7 yr old for games. This is what it is


Manufacture: Dell
Model: OptiPlex GX150 933
Standard Memory: 128 MB (removable)
Maximum Memory: 512 MB
CPU: 933MHz Intel Pentium III
No. Of Memory Socket: 2
Memory Type: PC133 SDRAM 168-Pin Memory

I know it says max memory is 512MB but it that right? Can I not put 2 512s in and use a gig?