Decisions, decisions...unsure on upgrade route

[Freakbro]

I work from home (accounts) on my old gaming machine and I'm acutely aware it's long in the tooth and not going to last forever.

Work Machine Spec :
Antec 900 Case
E8400 @ 3.60 Ghz
Asus Socket 775 mobo
8800GTS 512mb
4 x 2GB OCZ Ram

I'm debating whether to upgrade the work machine or use bits from the current gaming machine and upgrade that

Gaming Machine Spec :
Antec 1200 Case
i7 920 @ 3.80 ghz
MSI Socket 1155 (I think) mobo
AMD R295x2
3 x 2GB RAM (triple channel)
64GB SSD for OS

Will I notice that much difference going from i7 920 to a current i5 or i7 to warrant the cost?

I would keep the Gfx card, so just look for Mobo, CPU, RAM, new SSD or would it be more cost effective to just upgrade the work machine to something pretty bland, but that's still going to be faster than the current one.

 

[Freakbro]

I'm just concerned on the reliability stakes, I built 2 other similar spec machines at the time for other people and both the others have blown their mobo's over the last couple of years.

A new gfx card is not an issue, I have a couple of GTX470's or 7950's kicking around I can utilise