Yes, socket 604 boards are compatible with 603s. I wanted to get faster processors, but the price break seems to be right at the 2.4Ghz.
I bought my 2.4Ghz/533Mhz for $243 per processor. As you can see, you pay a lot extra for the 260Mhz difference... Don't forget to multiply the cost x2..
533Mhz Retail Boxed
2.0 = $218
2.4 = $243
2.66 = $294
2.8 = $351
3.06 = $484