Hello. In DIY PC, there are usually 7 sets of item(s) that comes
together.
1. Motherboard(mobo) and CPU
2. Ram
3. Graphics card(GPU)
4. Harddisk(HDD)
5. Power Supply Unit (PSU)
6. Casing
7. DVD Drive
As you seems to be an Asus fan, i have come up with 2
configurations that uses more Asus products.
But as you may know, asus is a good brand and thus the price may
cost more than other brands.
I will list down the configurations according to the "7 sets of
items" above for easy reference
Configuration 1:
1. Asus P8Z77-M Motherboard and
Intel Core i5 3470 CPU --- $ 422
2. G-skill RipJawX 1600MHz CL9 Kit RAM -- 8GB(2 x 4gb sticks) ---
$65
3. Asus GTX660 DC II 2GB DDR5 Graphics card --- $385
4. Seagate 1TB 64MB SATA3 Harddisk --- $91
5. & 6. Corsair Carbide Series 300R Mid Tower Casing &
Corsair TX650W Power Supply Unit --- $240 [Fuwell is selling
this bundle at this price]
7. Asus DVD Drive --- $30
Total cost about $1233.
I know it is a bit out of budget, because i feel that it is more
worth it to get the newer motherboard chipset with the new ivy
bridge processor.
Configuration 2:
1. Asus P8B75-M Motherboard and
Intel Core i5 3470 CPU --- $368
2. G-skill RipJawX 1600MHz CL9 Kit RAM -- 8GB(2 x 4gb sticks) ---
$65
3. Asus GTX660 DC II 2GB DDR5 Graphics Card--- $385
4. Hitachi 500GB 16MB SATA3 Harddisk--- $63
5. & 6. Corsair Carbide Series 300R Mid Tower Casing &
Corsair TX650W Power Supply --- $240 [Fuwell is selling this
bundle at this price]
7. Asus DVD Drive --- $30
Total cost $1151.
In this configuration i changed the motherboard to a B75 Chipset,
and changed the hardisk to a hitachi 500gb instead of a 1TB
seagate. The others remain the same.
The difference is about $82.
Both the set up uses Nvidia Graphics, and they are manufactured by
Asus(there are other brands available also).
SSD:
The 2 set up above does not come with SSD.
A Corsair Neutron 120GB SSD will cost about $169.
There are other models which are cheaper, like the Corsair Force 3
120GB for $131.
Future Upgrades:
Both the motherboard have 2 ram slots, so it is impossible to add
extra ram unless you change the current ones to those with bigger
capacity, i.e. from 2 x 4gb to 2 x 8gb sticks. However I don't
think there is a need as 8gb should be enough for normal use. There
are also motherboards with more ram slots but they will cost a
little more.
The config 1 motherboard has 2 x SATA 3 port and 4 x SATA 2
ports. You can still use SATA 3 hardisk in the SATA2 ports, but
they will not run at sata 3 speeds
The motherboard in config 2 has 5 x SATA 2 ports, and
only 1 x SATA 3 port.
If you were to take this board and intend to get a SSD, i think it
will be better to plug the SSD in the SATA 3 port, and leave your
storage harddisk in the SATA2 ports.
From what i see when repairing computers, some of the difference
between branded and DIY PCs are as follows:
1. Power Supply in some branded PCs are installed just nice for
that system's intended usage.
How it may affect us: Upgrading to a very power hungry graphics
card will not be possible unless we change the power supply as
well. However adding more hard disk should not be much
problem.
2. Some motherboards use in some branded PCs are custom built to
fit the PC specs, eg. only 4 sata ports.
How it may affect us: Adding a few more harddisk may be a problem
as there are no more ports
3. Not enough expansion ports in the motherboards.
How it may affect us: We cannot buy a sound card/tv card and plug
it in. Some do come with 1 or 2 expansion ports, it really depend
on the manufacturer
4. The casing is hard to dismantle/designed to fit that motherboard
only.
How it may affect us: We may have problem reusing that
casing.
Then again, branded gaming computers nowadays seems to be designed
to be easily upgradable....so it really depends on what you are
getting.
Also not all the sales person who sell the computers know the exact
part and model of components used in the PC. One of the reason is
they manufacturers didn't state it out, or they have too many
models of PC to remember 
.
This may affect us if we want to buy more ram from sim lim square,
and the sales person present us with many rams with different
speed, and we have problems deciding if the faster rams will work
with my current PC or not.