My suggestion would be to stick with the Northwood. Prescott overclocking is considerably more risky to i875 motherboards due to their large power draw, and as you mentioned you're not up for volt modding.
Selection of cpu may be balanced with what type of RAM you have. If you just have vanilla PC3200 RAM, you may choose something with a higher multiplier, such as a 3.2c, altho it may difficult to find one for $200. The lower the multiplier, generally the higher the headroom, but the need for faster RAM. For example, my 2.4c does a FSB300, unfortunately they don't make DDR600 yet.
Traditionally, the 2.4c processor have been good overclockers. The 2.6c not so good. If you can get someone to hand pick you (and smaller companies like iguanamicro or azzo), try to find a 2.8 SL6Z5. Also, try to request a "30 cap" 2.8, 3.0, or 3.2c. The steppings aren't consistent with getting a "30 cap" as far as I know--it requires that your hand picker actually look at the processor and find 30 caps instead of the usual 12.
Hope it helps,
navig
