Yes, the only way is to overclock the FSB (front side bus, or system bus as you said). That is of course assuming you are unable (or unwilling) to unlock the CPU yourself.
As for side effects: if you overclock your FSB, you're also overclocking your PCI and AGP busses. That stresses any cards in those slots. Some cards can handle significant increases (Netgear NICs can handle up to 40% increase, according to the link provided in a minute), and some not as much (3Com NICs would only go up 17MHz or so). There is a review at PCStats <A HREF="http://www.pcstats.com/articleview.cfm?articleID=937" target="_new">here</A> about overclocking NICs. It will most likely have some general information as well, although I have not read it yet.
In regards to that, most modern motherboards support dividers. If your FSB was at 100, then your PCI would have a 1/3 divider, placing it at 33 (which is what it's supposed to be at). If you raised the FSB to 133, you could (providing the motherboard supports it) change the divider to 1/4, placing it again at 33. Without the divider change, your PCI bus would be at 44. That would be handled ok by a good number of cards, but it's far easier to keep things in spec (at the speeds they're supposed to be at, in other words).
There is one motherboard (the Abit TH7-II; a socket 478 i850 board) that can "lock" the PCI and AGP, so that you may move the FSB freely and not affect the PCI and AGP speeds. Very nice.
If you have any questions about what I've said, you can reply here or (if you prefer) feel free to email/PM me.
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