Dell dimension 4500 need help with specs?

I have a dell dimension 4500. i was wandering what my limit per ddr2 ram is for my slots. i have 2 slots or 4 slots i dont remember. I have 1 512 mb ram and 1 256 mb ram in 2 slots now both ddr2. now i think i would be able to hold atleast 1 gb in each slot or maybe 2 gb cuz my dads motherboard isn't as new and it can hold 2 gb ram in each slot. and i have intel pentium 4 2.4 GHz so i was hopen to make my pc a bit faster and add another 1 gb ram with the same 512 mb ram and take out the 256. so in total i'd have 1512 mb ram. I was also wandering what graphics car i'd be able to go up to. i have a ati radeon 9200 series 128 mb ram ddr2. just to let you know i'm tring to make this so i can run my games better plus i want it to be faster. pls help me with my specs. oh and i believe i have over 600 fbs