I'm not sure I understand your question, I use 5 gigs because it's supported and that's the RAM I currently own.
I did try setting the HDD to boot before the DVD-ROM and it made no difference.
Edit- I removed my 2nd HDD which just stores my music and the system loaded windows with no issue. Can a 2nd HDD in the slave port with no OS cause a failure to boot? It's always listed last in the boot order.