Installed memory not displaying

I have two questions regarding my ASUS P3V133 motherboard:

1. I've opened my computer and see I have a ASUS P3V133 motherboard with a VIA VT 82C693A chip. However, when the computer powers up, it lists the chip as a VT 693A-596B. Are these two chips interchangeable? Would this discrepancy cause a problem?

2. In the ASUS P3V133 manual specs, it cites the memory capacity to be 1.5 GB, using 3 @ 512mb SDRAM. My computer had 2 @ 256MB SDRAMs and 512MB memory was displaying. I replaced them w/ 3 @ 512MB SDRAMs, and now, instead of 1.5 GB of memory displaying, only 786,432K is showing available. How can I make full use of the 1.5GB I have installed?

I would appreciate any suggestions! Thanks.
Christine