The upgrade from i5 to Xeon is only worth it if you have applications that can take advantage of the hyperthreading offered by the Xeon. For gaming purposes the 4460 is fine.
I use only SSD now with no hard drive. I'll never use a hard drive again. SSD is so much faster at everything. You get same fps in games but everything loads much faster including new levels and checkpoint saves. OS is faster too.
Wow sounds like a must, so you only use externals for storage? you must have a big SSD.
Will having an additional HD in the computer make it slower?
SSD only, is best for me since I don't have large storage needs. For most people a combination of SSD and regular hard drive works well. Put OS and important apps and favorite games on SSD and use hard drive for bulk storage and less important apps. A 250GB SSD will work for many people. I don't recommend anything smaller than 120GB.