Actually, you're right. The form factor might just increase it's price by 'bout a grand or so. Probably about 3-4 thousand, I'd say.
I dunno, if it's the same in Pak, as it is, here in India, but with AMD being more expensive, I'm gonna look at the following - (I'm giving you newegg links, you can get these from your local store).
CPU : E5400
Mobo : ASUS P5KPL-AM/PS
RAM : (2 X 1 GB) or (2 X 2 GB) depending on your budget. Kingston & Transcend are prolly the way to go. Stay away from Tamkor etc. if you can.
HDD : Seagate Barracuda / Western Digital Cavier Blue / Samsung Spinpoint F3 (Dunno if they sell that in Pakistan, though). Go with 320, if you don't need a lot of storage - it'll be a bit cheaper than 500.
You've gotta get a half decent PSU to support a gfx card. In India, we'd prolly go with Frontech 500-600 watts. It'll come with the rest of the system anyways - no extra charges. Go with an inexpensive mid tower case.
For gaming you need a discrete GPU, & that's gonna be the killer, monetarily. Ideally you'll need the Radeon HD 5770
But at least the Radeon HD 4650
is the bare basic requirement.