Honestly, you're better off just replacing the 166mHz RAM stick. If you do want to try to use both sticks, I'd suggest placing the 333mHz stick in slot 0 and the 166mHz stick in the next slot.
 
I don't think that you are at risk of frying your motherboard, but you should probably wait to see if anyone else has an opinion on this. I personally have never tried installing RAM of different speeds into the same system.

thanks for your help. I did find something on the PCWorld site about mixing RAM of different speeds and they said there was no problem except that the RAM would run at the speed of the slowest stick. FRom that I assume I can but will not get the performance boost that I'd get with sticks of the same speed and manufacturer. I'll wait and see if anyone has anything else to offer on it before installing it.

In theory, mixing RAM of different speeds, as long as they are supported by the motherboard, is not an issue. I have done this, but saw the lack of overall speed difference. The issue is with mixing different brands and speeds, as in your case. This can cause unpredictable results, since not all RAM is created equal between brands.
 
I would try the best (faster) RAM first, then is that works fine, try adding the second stick. Your system will complain with bios beeps if there is a problem. Also, if you get unexpected crashes or errors, you have your answer. So the rule of thumb is;
 
1. If the RAM brand and type is supported by the motherboard.
 
2. RAM of the same brand and speed is preferred.
 
3. Take chances with RAM of differing speeds (ratings) and of differing manufacturers.
 
I've seen RAM of specific brands cause problems in some systems. Your best guide is the manufacturer's web page and if they have a compatability database of supported brands and types on the support page.
 
Of course, if it works...then....

To make life a lot easier on compatibility issues, you can visit www.crucial.com (a computer hardware site), it has a scanner on it that will scan your system and tell you what you have, and then list out products (including RAM) that you can safely put into your PC or Laptop. You can also purchase these items in the same step. Awesome site, and I use it a lot...would recommend it to anyone.

Thanks so much for all the info. I just checked back in after running the different brands and speeds of ram for 3 months and they work great together as far as I can tell.
According to Lavalys Everest the installed memory modules are:
DIMM1: Hyundai HYMD264 646B8J-J 512 MB PC2700 DDR SDRAM (2.5-3-3-7 @ 166 MHz) (2.0-3-3-6 @ 133 MHz)
Field Value
DIMM2 GB Micro DDR 512 MB PC3200 DDR SDRAM (3.0-3-3-8 @ 200 MHz) (2.5-3-3-7 @ 166 MHz) (2.0-2-2-6 @ 133 MHz)
 
I went to crucial.com at Kain's suggestion and it for some reason doesn't distinguish between the 2 sticks and just recommends an upgrade to 2G instead of the 1G that is there.
 
Thanks again for all your help you guys are great!!
 

I have seen problems due to mixing brands of RAM. Sometimes the system works better with just one stick, than using mixed brands, and I mean the system can run faster too. Sometimes you get away with mixing sometimes you don't. Memory problems are often hard to track down so I never mix; in the old days memory was expensive so we all tried mixing because we had to strip memory out of defunct systems to keep the costs down.