Yeah, the 3600+ is a faster CPU. On that note, the Brisbane @ 1.9 GHz is faster than the Windsor @ 2.0 GHz, so, when you're choosing between the two, try to get the Brisbane if it's available.
That said, something in your post struck me: You say that your motherboard will not support Core 2 Duo, implying that it would support a Pentium D 820. If I understand you correctly that means you already have an LGA775 system--which in turn means that, at worst, you're running a Celeron D 325 (2.53 GHz). If that is the case, I wouldn't bother upgrading if you can't do better than an X2 3600+. Just wait until at least you have enough to get an E6320 or better.