well its not so much the number of cores on the CPU for running one thing fast, the more cores you have the things you can run at full speed but if your just running one thing then you have 3 cores waiting to do stuff...
That said i would suggest an i5 over an i3 because even if you dont need all these cores today you might tomorrow.
I would recommend 8GB of RAM, again it might be a smidge more than you need but will never hurt to have more, (also get good quality RAM not 'value' RAM)
if your gonna be storing loads of music and movies then a normal HDD would be better, again good quality not the cheapest... I doubt you would need an SSD for this kind of oporation and that will bump the price up.
im not much of a gamer so im not to up on Graphics cards but i spent about £70 on mine and its not struggled on a game yet (does struggle on some high end bench tests though (like the one that came with it?!?!))
the mobo is often where peopel scrimp out, and although i am a beliver in getting things on cards (dedicated audio card, dedicated LAN card etc...) you should put some good money into the mobo. Asus are good, and i love Gigabyte, they make it real easy to over clock (so will the i5) and mine has a duel bios so in theory i cant brick my PC if i muck up the BIOS (dont see how i could, i can overclock my CPU from a 3.1 to a 4.9 in 2 clicks and a reboot)
*also make sure it has loads of USB ports, these will always end up being in short supply.
get a nice sized case as it will make it easier to work in and ensure you have fans in the case and fit them correctly to allow good airflow (especialy if your overclocking (you might even need a bigger heatsink and fan on the CPU but my stock one is coping))
PSU, this is another thing people offten scrimp on, make sure you get a beefy one, i think mine is a 1000W and its modular so you only need to have the cables you need coming out of it.