Can i possibly run diablo 3?

Windows® XP/Vista/7 (latest service packs) with DX 9.0c
Intel Pentium® D 2.8 GHz or AMD Athlon™ 64 X2 4400+
NVIDIA® GeForce® 7800 GT or ATI Radeon™ X1950 Pro or better
1 GB RAM (XP), 1.5 GB (Vista/7)
12 GB available HD space
DVD-ROM (required for retail disc versions only)
Broadband* Internet connection
1024x748 minimum resolution

your gfx is ok but your cpu is to weak. it will install ok and probably play fine right up untill you meet another few npcs then your gonna see your system crumple...

best consider upgrading that cpu and motherboard... a cheap dual core like the i3 2100 would be great and for under 200 pounds you would see a huge improvement across the board with your pc...
if your motherboard is a 775 socket and can handle a C2D then a 2.4 or better would do the trick but your likely to pay over the odds for a new 1 so maybe look for a refurb 1 instead...

Dang, I was sooo hoping it might work  

it may work but i would recommend you download a playable demo first if there is 1.
but as i say its likely to crumple as soon as a few beasties appear on screen... but you never know till you try... crysis for instance ran better on a p4 3.6 than on a pentium dualcore 2.6 as long as the gfx didnt bottleneck... but even though your gfx is pretty weak it still may be to strong in some cases for your cpu...