Hi. I'll give this a go as you have not had a reply yet.
Your motherboard, the jetway 663as pro has a maximum fsb of 100Mhz so you are slightly limited in what you can put in. The manual says that 1,000Mhz thunderbird ( 10 X 100Mhz) cpu should be the fastest available. But there is evidence that 1,300Mhz or 1.3Ghz cps have been used.
This is with bios rev.1.2
So it would seem that a clock multiplier of 13 and a fsb of 100Mhz is possible. This would lead to a duron 1300 may work with the right bios revision. Thunderbirds are very difficult to get hold of and the duron 1300 maybe easier to locate. Info on duron click here=
These are getting difficult to get also, but they are some on ebay.click here
One option would be to get a faster fsb cpu and run it at a lower fsb.
A 1600 duron ( fsb 133 ) would run at 1.2 gig because the clock multiplier is 12 and you could only run it at 100Mhz. 12 X 100Mhz = 1.2 Ghz. While it true speed is 12 X 133 = 1.6Ghz.
Also a palamino 1900+ should run at 1.2gig because it has the same specs as the duron. 12 X 133. click here
One large advantage of running a faster cpu at a lower fsb is that the cpu temp is lower.