What you allready have ? ( monitor , what resolution, keyboard,mouse , OS)
I don't remember my monitor, I'll tell you when I get home. I run Windows 7 64 bit and have a pretty standard kb & mouse. I can understand the monitor but does the other stuff matter too much? Just wondering, I'm kind of new to it.
Opinion: Having a beefier PSU will help if you choose to X-fire later. The SSD will make you productivity use much faster and pleasant as will the far superior CPU.
Having USB 3 front connectors is surely a must for a new build these days. Hence case selection. If you need more storage I suggest pulling out your current HD or finding a cheap deal at a later point. HD's are still overpriced from the flooding to the point I recommend buying second hand or waiting until they drop again, SSD's just offer too much in comparison.
GPU wise I would say the performance between the 7870 and gtx580 is much of a muchness, but power draw and temperature of the 7870 is far superior. Also possibility of multi monitor gaming is a big draw as it is now realistic on a single GPU like this.
CPU:Intel Core i5-2400
CPU Cooler: Intel Boxed Heat Sink and Fan
Motherboard: MSI PH61A-P35
RAM: Wintec AMPO 4 GB (2 x 2 GB) DDR3-1333 3AMD31333-4G2K-NHR
Graphics: Sapphire 100314-3L Radeon HD 6870 1 GB
Hard Drive: Seagate Barracuda ST3500413AS 500 GB SATA 6Gb/s
Case: Xigmatek Asgard II B/B CPC-T45UC-U01
Power: Antec EarthWatts Green EA430D 430 W
Optical: Samsung 22x DVD Burner SATA Model SH-222AB
Thats the $600 build posted on tom's buyers guide which comes to roughly £530. To increase to £800 you can now add a better GPU and get a better powersupply. Try the 6950, or wait until the 7850 which comes out late March/early April. If you are going to get a better GPU i'd suggest a PSU around 500-600 watt, make sure you get a reliable manufacturer (antec, corsair).
Because you're looking to watch a lot of films you could upgrade your hardrive to 750gb's or 1tb but this can always be done at a later date when prices fall and when you fill the 500gb one.
Many people will state that you'll want to upgrade your motherboard to a H67 or a Z68 however i don't believe you can afford it at this price range. Sticking with a i5 2400 as i don't think you'll want to be OC'ing so there's no point paying the extra £35 for something that will hardly make any difference.