Okay heres what I came up for ya.
Evga NF-63TR board. $159.99 This board is a great core2duo overclocking board. It will work with quad cores but will not overclock them. Your not going extreme on the build thats why I suggested this board. You would need to make sure you update to the P-30 Bios for the CPU I have listed. This board will not support future 1333FSB quad cores only dual core systems.
E6550 core2duo $175.99 with the 1333FSB make sure you update the bios to P-30 for the 1333FSB support.
G.Skill 2X1 gig PC6400 low latency $99.99 This ram owns
XFX Geforce 8600GT, This graphics card will offer you full DX10 support for Vista. Not a high end card but should play games on medium/high settings
This PSU should provide more than enough juice for you the Antec 550W high effeciency $99.99.
Total works out to $675.95 minus the $20 Rebate on the GPU so its really $650.95. You really wouldn't need to upgrade the HSF unless you plan on going for ultra clocks. With that board/ram/cpu combo you should hit around 2.7-2.8Ghz with the stock cooler.