Smithfield chips have two 90nm Prescott cores on a single die, with 2*1MB L2 cache. They are labelled as Pentium D 8XX chips
Presler chips have two 65nm Cedar Mill cores on a single die, with 2*2MB L2 cache. They are labelled as Pentium D 9XX chips.
Because of their smaller manufacturing process, Presler chips will run cooler and draw less power than similar Smithfield chips, so look to those if you want to build an Intel system NOW. However, the Pentium D 805 is a good Smithfield option at around $120. It offers excellent overclocking abilities when paired with a good cooler.
Conroe, on the other hand, is not based on the Netburst architecture like the two aforementioned cores and all other Pentium 4 chips. Its architecture is more related to that of the Pentium M/Pentium III. Early benchmarks and tests have shown that this core is incredibly fast, often beating out comparable AMD chips by a significant margain. It will come in several varities on July 23rd, with some carrying 2 MB Shared L2 cache and some carrying 4 MB. Prices will not be much higher than even the slashed prices of current Pentium D prices. I would say that a Conroe chip is your best option for building a new system. So if you can wait a few weeks, a great new chip will be released at a very reasonable cost, Pentium D prices are likely to drop even further, and AMD will also cut prices on their chips soon after Conroe's release.