Hmmm I would say if it's a thunderbird Athlon then it might go to 1Ghz, there were some thunderbird cores on the later slot As. Edit: Doh, if it's 512K it's not a thunderbird of course.
However, that would involve fighting the RAM too if you want an FSB overclock since it's PC100. Though most can push to 124Mhz when it's proper 8ns PC100 spec RAM.
However, it kinda sounds like your video is onboard too, with you saying about shared memory architecture
However, if your case is a standard ATX, I've come up with a sub $150 upgrade suggestion that might fit the bill, it could be up to 3x as fast as your current rig, but it's not the best stuff, for squeezing it into a low price like that...
http://www.geeks.com/details.asp?invtid=MK40VN-N&cat=MBB
Shuttle miniATX mobo with onboard graphics. (plus an AGP slot for later) $33.95
http://www.geeks.com/details.asp?invtid=DHD1800DLV1C-N&cat=CPU
1.8Ghz duron, give it a little more voltage and it will run at 166 for 2.2Ghz .... $45.50
http://www.geeks.com/details.asp?invtid=CP5-8J71F-N&cat=CPU
Aero7 lite cooler, not top end, but will do the job .................. $14.00
http://www.geeks.com/details.asp?invtid=AP450W&cat=CAS
Cheapass 450W PSU, should be better than current one and be "good enough" $14.00
http://www.geeks.com/details.asp?invtid=256DDR3200-N&cat=RAM
256Mb of PC3200 DDR, not much but upgradeable later... ... .... .............. .... $39.00
That all comes to a total of $146.45 and is the kind of thing I might build for my sister in law or someone, not an awesome overclocking rig, but should be capable of some fairly recent games, and wouldn't choke on the newest by the time you get a graphics upgrade and another stick of RAM in it. The unichrome graphics are "meh" but not any worse than the radeon 7500 I think, and get you going until you can afford a decent card. A point to not however, is that since it's mini ATX it doesn't have many PCI slots, you can only practically use 1, but if you're satisfied with the onboard sound and LAN, then there's not a lot else you'll use it for anyway.
Anyhoo, IMHO it would be a better $150 worth than the $150 you could spend upgrading the CPU and RAM on that motherboard, for only maybe 25% more speed.
regards,
Road Warrior
edit: of course if your AIW isn't onboard, you can try that in it to see if it works out better than the unichrome onboard.