well, i guess you can overclock it up to 166Mhz ... safely with VCore 1.7 or lower. However, you should increase you FSB by 4Mhz at one time and test the system. (4Mhz and later 1Mhz. ... with appropriate increase in VCore.) and watch out for cpu temp. If it runs too hot (like over 60C), it's better not to overclock ... alot of heat will degrade your performance. (ye, i was suprised at the benchmarking after i lowered my cpu temps by 10-12C ... the benchmarking was obviously better. It was testing memory bandwidth.) It's a good idea to buy a new cooler if you want to overclock. (i bought vantec aeroflow and it cooled down my cpu temp by 10-12C) Not sure if you can overclock FSB up to 200Mhz ... it'll be a big leap from 133Mhz FBS.
However, i'm not sure if you can overclock you mem freq. too ... In my case, since I have OCZ PC3200 (200Mhz) memory, i had no problem in increasing (not really overclocking) my mem freq. (the mem freq was originally 166Mhz to keep 1:1 ration with CPU FSB.) If your 133Mhz is standard mem freq, (which means you have PC2100), then i can't say if you can overclock it. (If you mem is one of those well-know brand for overclocking such as corsair or OCZ ... then i think you can overclock it.)
I sympathize. I have a KT133A system as well (Epox 8KTA3PRO). I haven't found many gains from overclocking. My system performs about the same at 12.5 x 150 Mhz = 1875 Mhz as it does at 15 x 140 Mhz = 2100 Mhz, the latter being only slightly better. I can push my system to 2267 Mhz (17 x 133) but there are even fewer gains plus I have to push vCore to 2.15 volts. (Can you say "HOT"?)
I don't think you are going to find much more performance with PC133 memory but if you have an unlocked Tbred B you could try a higher multiplier. You've got a much better video card than me. Even the smallest gains might help you.
If you don't have an unlocked processor than FSB is your only hope. It's likely you are at you limit with your PC133 memory but you never know. One of my sticks is PC133 (the other is PC150) and I have it at 150 Mhz. I thought I was limited to about 142 Mhz until I pushed I/O voltage/memory voltage to 3.6 volt. I also lapped my northbridge heatsink for better cooling. It wasn't flat and the contact point was only about 3/8 inch round. I've been running the higher voltages for 2.5 years now.
If your mobo limits your vCore to 1.8 volt you can do a bridge mod on your CPU or a simple wire-in-socket mod (my favorite) to get you up to a max of 1.85 volt. You'll need good cooling.
For the record, what kind of performance (framerates) are you getting in Doom 3? Is it playable at 800x600 on your rig?
<b>56K, slow and steady does not win the race on internet!</b>