More memory vs. higher bus speed

I was a computer guru about 8 years ago, but I have been out of it for some time and now have lost all the knowledge in my head. Anyway, I am building an arcade for my son. Computer is a Dell 8400 with single core 3.2ghz cpu and originally 512mb pc2-3200 ram. Needed ram upgrade badly. I purchased 1.5 ghz of working ram pc2 5300 at 667 bus speed. 3 sticks of 512.
My options are to use 3 sticks of the 667 and one 400, for a total of 1.75 gb, but will only sit at 400 bus...
or I can just use 1.5ghz and keep my 667 (if my board will let me set single modules) Im not certain on that yet. If not then my last option is to just have 1.0gb of ram at 667 bus.

Anyone have any suggestions as to what may work best for me.

Each memory slot can hold DDR2 PC2-5300 with a maximum of 1GB per slot. I do not want to put out any more money than I already have. 1.0gb of ram should be enough to run the basic things I need, but If I can get more speed out of what I have, why not use it? 512 just wasnt cutting it.