Well with an introduction like that I hope I dont disapoint
Right - go quad as Polo has said, have a look at the post first build of his, as for MSI, I personally hate MSI and would opt for ASUS anyday.
RAM is ok.
You have listed an ATA drive - go for SATA as they are faster -
260 GTX is a great card (have one myself) but in terms of value for money at the moment you want an ATI 4870 1GB (these are around £220 now) and blow the 260 away.
As for a case have a look at the Antec twelve hundred and yes Vista is far better although its risen in price lately.
For £600 you are going to be hard pushed to get a decent gaming rig.
you are looking £300 for the CPU, RAM and MOBO therefore cases and GPU's are limited and you cannot house the spec you want in an old beige case.
What I would call a mid level gaming rig is easily nearer to £800
If £600 is your absolute limit you are really going to struggle to get a decent gaming PC.