I don’t think Biostar is a brand I’d look for if you’re really looking to OC. As I recall the chipset that’s utilized in your motherboard (the VIA P4X266) is a single channel solution, and you would gain some performance by going to an i865PE board, which has the ability to lock the AGP and PCI busses, and the P4X266 does not. This, besides whatever voltage adjustments you may or may not have, and that your RAM is only spec’d to run at 266MHz/133MHz (sometimes the latter in your BIOS)
Quite honestly, you’ve got a 36.5MHz PCI bus as is (divide your FSB by 4, which currently equals 145MHz and then that result by 4, as your PCI bus is determined by dividers with your chipset, and would b 4 in this case, and equals 36.5). The thing you’re looking at as a problem isn’t the CPU as much as the RAM (which, if it’s good could hit 166MHz, or PC2700 speeds), and the PCI divider. You start getting up around 40MHz on the PCI bus and you’ll start seeing your PCI devices (on-board and PCI cards) start to not show up. You’re also asking for trouble as your IDE controller is linked to this and will start causing corruption at this point in time.
However, for what you do, I really can’t say overclocking will really do much for you—at least to the extent you’ll notice. If you were more into FPS gaming, I’d look at a video card upgrade, but unless you’re noticing some serious lag in NFS and/or playing at a high resolution (1280x1024 and up), I’m not sure a video card upgrade/overclocking would do much for you.
Well if you don’t care too much about screwing up your system, you could just over clock it until it fails to boot, and then knock it back a couple of Mhz!
Maybe up the RAM to a gig, and get a 9600XT for a decent budget card.
The processor is fine for the time being, I would say.