Nice call on the graphics card, I will definitely have a look at that (although the case only comes with a 430W psu I think it should be ok with that, I don't plan multiple hard drives or anything since I want it to be as economical as it can be for a 24/7 folding PC)
As for the motherboard, what you state about application is what I thought. BUT then I read some reviews, the most thorough of which was this one..
and as shown by most of the benchmarks in this test, applications that do utilise multiple cores were marginally faster than the 8400 (but very marginal at that) and those that are single threaded was substantially faster with the 8400.
The most relevant of these to myself will be divx and avi encoding/decoding which, as shown here http://www.neoseeker.com/resourcelink.html?rlid=170757
(see drop down page list - divx encoding) , is about 80% faster under 8400
But again, as you point out, faster RAM would benefit the Phenom, and I'm sure the AMD chipset present on the specified MB would also sway it somewhat. Either way they are both better than my current spec (4800+ X2) which isn't exactly slow at divx coding so I may just settle for the phenom. I'm not sure whether its idiocy to be this loyal to a company 'just because' but I live in hope one day things will be on par again. AMD64's ruled for 5 years, now its Core 2's turn.
EDIT: I just realised (rather poorly I know) the reviews CPU is the 8750 not the 8650 which can only mean the 8650 is even lower
I shall have to reconsider budgets here as there isn't much point getting anything that isn't going to last at least a couple of years.
As a closure to this post I would like to thank worshipme for his contribution and the final (purchased) spec list is as follows:
AMD Phenom X3 8750 - £123.38 (scan)
Toliman Core, Socket AM2+, 2.4GHz, HT 3600MHz, 3.5MB Total Cache, Retail
Asus M3A78-EMH HDMI - £51.35 (scan)
AMD 780G, S AM2+, PCI-E(x16), DDR2 1066/667/800, SATA II, SATA RAID, HDMI, µATX
512MB Sapphire HD 4850 - £128.02 (scan)
PCI-E 2.0 (x16), Mem 1986MHz GDDR3, GPU 625MHz, 800 Cores, 2x DL DVI/HDTV
500 GB Maxtor STM3500320AS DiamondMax 22 - £46.87 (scan)
SATA 300, 7200 rpm, 32MB Cache, 8.5 ms, NCQ
Corsair TwinX £40.64 (scan)
2GB (2x1GB) DDR2 XMS2 Dominator, PC2-8500 (1066), 240 Pin, Non-ECC Unbuffered, CAS 5
Antec Fusion - £111.03 (scan)
micro ATX case with 430W PSU
Lite-ON LH-20A1S-11C - £17.61 (scan)
LiteOn 20x DVDħR, 8x DVDħDL, DVD+RW x8 / -RW x 6, DVD-RAM x12, SATA, Retail
Total = £518.90
Whilst slightly over the 'budget' (pretty much just the lowest amount of money to spend which gives long-term performance, compatability and extendibility) I think this should be a more than competant machine for any task, and certainly media streaming + folding.
For anyone thinking of building for a similar purpose I hope the information in this thread helps. (p.s. anyone looking for media pc cases that hasn't already seen the antec fusion, check it out)