Alright, so you can't just increase your fsb multiplier on non-Extreme Intel CPUs. But can you DECREASE it? I would just reboot and check myself, but I'm downloading some stuff right now and its not a torrent so if I restart, its from scratch (stupid ten gig demo software).
The thing is I have Windows 7 Ultimate here in my hands, and would like to try out XP mode as well, but I need a processor with hardware virtualization to do that. Ordering a new dual core LGA775 part isn't very expensive, but some or most of the processors that support Virtualization under $100 use half multipliers, like the E5400 which uses a 13.5 multiplier. My motherboard is an (now ancient) Gigabyte P965-S3, which doesn't support (I believe) half multipliers. If I can't lower it to 13, then I'm out of luck.
Thats another good point: even if I can lower the multiplier, will a processor that uses a half multiplier even be recognizeable to my motherboard? I'd imagine so, but does anyone know for sure?
You can decrease the multiplier (Intel SpeedStep does that), but make sure that the CPU is on the list. Instead of an E5400, have you considered the E6500 with a multiplier of 11? It only costs $5 more at Newegg.com and it also is supported by your motherboard.