Hugo 59 gives a great overclocking 'diagnostic' outline to follow - you have to troubleshoot the problem before you can fix it.
going back to what you said about your network card, if you think it might be holding you back, take it out and see what happens. i would strip the system bare with only 1 ram stick, graphics card, and HD and build on that. if you can oc higher than your previous setup, than you know one of your components is holding you back. if you can't, at least you ruled out most of your choices so you won't lose sleep thinking about, "is it my memory? is it my NIC?".
and about not wanting to take out any ram ... i was in the same position also. it was either 146fsb w/ 512 or 153fsb w/256. i figured to hell with the 512 meg and i went with the extra 7fsb. even if i went with the 512 meg/ 146fsb combo, i knew that if i ever wanted to run a higher fsb, it all starts with removing a stick of ram and going from there. (now its my agp holding me back

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try what hugo 59 says .. its a good start!