I got four 0.5gb modules that are recognised by bios.

I have two 2gb modules that are not but win8 displays 5gb but 1gb usable.(i lose 1gb by replacing two 0.5 by 2gb)

Computer is hp xw6200 and supports 8gb max.

Modules as below
512 mb 1rx8 pc2 3200r-333-12-ao
Ddr2 400 cl3 ecc rgg

2 gb 1r4 pc2 5300p-555-12-ho
Ddr2 667 cl5 ecc rer

Im guessing its rank issue: 1rx8 vs 1r4