first, if you want upgrade-ability, you either need to wait for the upcoming intel sandy bridge stuff (assuming that socket lasts more than a year), which is hopefully still on target for jan-feb 2011 release or thereabouts,
...... OR go AMD, something like a phenom 2 x4 955, or the new x6 (1035t, 1055t, 1075t or 1090t black edition) on an AM2+ or AM3 motherboard.
if future upgrade isn't a main priority, then an intel i5 makes for a great system (I've built a few for mates gaming rigs).
for gaming intel has a marginal lead, but for the other uses (photoshop, video encoding. I have no idea about reason) AMD x6 has the edge.
If you plan to overclock, intel generally yeild a bigger increase, although AMD is narrowing the gap.
WOW apparently favours intel, and higher clockspeed = better, as it can only use 2 cores properly. also bare in mind that unless you do amazingly complex things in photoshop or reason (or really cant stand waiting 2 seconds for a filter to apply in PS), then gaming will be the only taxing use of your PC (I use reason 3 on an old core2quad and never see more than about 15% CPU use)
RAM: on either system, you want 4gb (2x2gb) of ddr3, the 1600mhz stuff is probably the best value for the performance.
£80-£100 is plenty
graphics card: WOW and COD also favour nVidia, so your best bet is a 460gtx, plenty of grunt, nice and cool, and don't use too many Watts!
hard drive: samsung spinpoint f3 1tb (terabyte); will take ages to fill unless you download lots of vids, and you can add another later if u want for: RAID1 for security, or RAID0 for speed. ~£50-£55
power supply: look to spend
£70-£100 on this part,
cheap PSUs are just plain bad! had one die on me recently and it scared the poo outa me, but luckily everything survived.
you'll need something around the 650w-700w mark to ensure future upgrade, and preferably modular (meaning any unused cables can be detatched for better airflow). Corsair make excellent PSU's with a very good rep for reliability. (which is partly why my new PSU is a corsair HX850

)
add to this a DVD RW drive (<£20) and whichever case takes your fancy, and you're away. i will also add that these days, windows 7 is considered a must for gaming.
specific parts I would look at would be
this graphics card
this PSU or similar
if you want to buy a preconfigured bundle (CPU+RAM+Motherboard+cooler)
your 2 main options are
this intel bundle or
this AMD bundle
however if they are stretching the budget a bit you could get
this bundle and upgrade to a x6 CPU later on
If you want to save yourself the bother (and fun) of building it yourself,
look here! but remember you will need to add the cost of windows.
also, you want the x64 version of windows 7, as the 32 bit version will make some of your ram unusable. (applies to systems with 4gb or more plus a gfx card)
hope all this helps.
Trojan
P.S. If you use very large (high fidelity) VST plugins in reason, or photoshop MASSIVE images (like 7000x7000 pixels or so) then your next sensible upgrade will most likely be more RAM, although if you use PS CS5 i think it can do most things on the GPU (graphics card)