Actually, there may be something for you, but I cannot confirm it.
When LGA775 replaced socket-478, there were some decent ASUS boards which paired LGA775 with DDR memory.
I thought I came across some more recent examples of such boards while shopping for the parts in my "Double-Whopper" build -- running Orthos as we converse on this thread.
I didn't pay a lot of attention to those items. As for pairing a Core 2 Duo with DDR, this is not something that would be unfeasible -- it would actually be reasonable to find something like that in a cheaper motherboard. And by cheaper, I mean fewer options, less "bling," perhaps even with PCI-E graphics but not enough slots for SLI, possibly something in a micro-ATX form-factor.
Without recalling what your real options are, since I wasn't paying attention when I came across the board specs at reseller sites, I'd say your chances of building a decent but modest Conroe system -- even with DDR DRAM -- are high. But I would stick with the processors which are limited to 800 FSB -- E6300, for example.
As for over-clocking, it depends on how good the DDR400's are. Are they "low-latency-elastic" modules? Do they have really tight latencies that do not require loosening in order to over-clock maybe an additional 36 Mhz (DDR)? Or are they high-performance DDR500+ modules that can run at tighter latencies at lower speed, or maybe something like [3 or 4],4,4,8 at 533 Mhz?
Now . . . . I'll feel foolish if my thoughts here are proven wrong, but I do believe I saw some boards "out there." If the manufacturer has a good rep, and you don't find any product reviews pointing to failed quality control or unpredictable deviation from specs, it might work for you.
And does the budget-motherboard have a decent BIOS with some over-clocking options?
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