First of all
Next question that anyone has to ask - what are you planning to use this build for ? I'll try to help you evaluate the differences between these boards without knowing... First of all, they BOTH support 16 Gb Max Memory
Asus P5Q SE PLUS, as you note, newer and uses DDR2 1200(O.C)/1066. Mostly I see that it has no RAID Support (no big deal for a typical user).
Seems to want to use slower memory...but it would work just fine.
ASUS P5P43TD, DDR3 1600(O.C.)/1333/1066/800, no RAID support eiher. Seems like a decent choice, best price.
ASUS P5P43TD Pro
, again DDR3 1600(O.C.)/1333/1066/800, SATA RAID onboard, south Bridge uses ICH10R (not ICH10 - newer chipset ?), also offers eSATA on back panel. Not much more money, looks like the prime choice - although you may never notice the RAID, etc. unless you like to tinker... I'd get DDR3 1600 even if I didn't want to OC... and Newegg has an Open Box on sale (act fast if you want it) for $66 - cheapest of all...
http://www.newegg.com/Product/Product.a ... -_-Product
I'd suggest Min of 4GB, perhaps 8Gb (4x2), check product reviews for a nice 750w PS - for CPU - I have no idea of your budget - so:
$260 - Intel Core2 Q9400S Yorkfield 2.66GHz 6MB L2 Cache LGA 775 65W
Quad-Core Processor - Retail (6Mb Cache)
http://www.newegg.com/Product/Product.a ... 6819115209
$260 - Intel Core 2 Quad Q9550 Yorkfield 2.83GHz LGA 775 95W
Quad-Core Processor Model BX80569Q9550 - Retail (16Mb Cache)
http://www.newegg.com/Product/Product.a ... 6819115041
or - if money does matter:
$150 - Intel Core 2 Quad Q8200 2.33GHz LGA 775 95W
Quad-Core Processor Model BX80580Q8200 - Retail (4Mb Cache)
http://www.newegg.com/Product/Product.a ... 6819115055
About $100 for 4Gb Memory - CORSAIR XMS3 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model CMX4GX3M2A1600C9 - Retail
http://www.newegg.com/Product/Product.a ... 6820145260
I gotta figure out when to quit - that should be enough said...