This is a new issue with vegas I have all the same as you technically BUT I have 8gb of ddr 2 ram. I have been having problems with this after the update of vegas pro 9.
I have made loads of checks on my pc and its running great, i wonder if anyone has reported this to snoy andway here are my pc specs:
intel q9550 2.8 ocd to 3.2ghz
cpu at 400 with fsb at 1600
ocz gold series ddr2 at 400mhz 5,6,6,18 ratio 1:1
gigabyte p35c-ds3rv2 motherboard
bfg tech gtx 260 oc max graphics card
2 x wd raptor 150 gb raid 0
coolermaster 850 modular psu
coolit water cooling.
pioneer blueray writer.
Focusrite firewire saffire pro 24 audio interface
edirol monitor speakers front
creative gigaworks speakers rear
centre speaker not yet installed
iiyama prolite e2403ws 24 inch hd monitor
samsung 2433bw 24 inch hd monitor.
vegas pro 9
cinescore
soundforge 9
acid pro 7
sonar 8 pro
vista ultimate sp2 64 bit
advice would be helpful on this issue its driving me mad too.