Help Help Memmory Discrepencies

At boot the memmory test reads the full 512MB and then some but under Windows reads 511MB.
What give's?
Under Si Soft Sandra the readings only read 519 & 525 respectfully.
I'm running a 1.4Ghz. AMD OCed to 1.432, with the right code, Abit KT7A Raid and 40Gig.WD hard drive using Win.98.
If I OC more system tends to give VXD warning message.
Is it the Win98 causing the problem or could it be something else.
I turned off scanreg because it was giving no end of headaches and I have had fewer probs.
When running SiSoft memm test it will freeze or give VXD warning message but after I press a key will give a reading,usually, but after the SiSoft looses video definition. I'm using an Assus 7100 GForce 2. The system seems to run well with the tweaks I have made with the register and auto & config but has trouble with OCing. I changed memmory and system has fewer probs.
So what gives?