What is your partner running atm and does he build his own PCs? It may be worth finding out what he has and using an upgrade path if he has one planned. It may ruin the suprise but if he was like me I would like to know whats going on; I think more planning goes into my PC than my family.
With the overall outlay an SSD would be a must in my eyes.
The SB 2600k chips are supposed ot be on a par with the hex I7s in terms of performance.
Ram is really cheap atm, upgrading to 12gb may only add £50 odd quid.
I imagine your HDD are going to run in RAID0; a backup HDD may be required.
Your interent speeds are mad you could always try for online storage as you upload is faster than my download.