The E6300 has an outstanding overclock potential. Its default frequency is 1.86Ghz and people have pushed it as far as 3.4Ghz (in tests). Stable overclocks (for everyday use) reach to 2.6-2.8Ghz easily.
The case is ok, but you should know that included power supplies are usually low quality. That means it's more probable it fails within the first two years, in which case you'd have to buy another one. The real problem, however, is that when it dies, it can take another part of your computer with it.
Now, I've used case-included power supplies for a long time (no money to buy good ones), and although they do tend to fail, they've never damaged any other component. Maybe I'm lucky, I don't know.
So basically, it's a risk. But since you're on a budget, it might be worth considering; just keep the potential consequences in mind.
As for the motherboard, I'm not sure about the one you proposed. It has an nVidia chipset, but Intel chipsets are better for overclocking Core 2 Duos. The only reason to get a board with an nVidia chipset right now is if you want SLI, but it doesn't fit in a budget PC (and it's not even worth it anyway).
I upgraded the motherboard to the next category anyway. Read further.
I'm glad you have a monitor, because that gives us some extra $150 to work with.
I started by upgrading the RAM to 2GB (instead of only 1GB) of faster DDR2-800 RAM. It even comes with heatspreaders, so it's ready for overclock.
With the faster RAM I had to upgrade the motherboard as well. I suggest the Gigabyte S3. It's one of the best Core 2 Duos motherboards out there; it has tons of options and is excellent for overclock. The price is also very reasonable.
Finally, I upgraded the video card to an X1950Pro (512MB version). It should easily outperform the 7900GS we had initially conceived.
Here's the updated build
. I decided to keep the Fortron power supply; it's reliable and powerful enough (two 18A +12V rails). You can save the one included with the case for an emergency.
: Intel Core 2 Duo E6300 1.86Ghz
: Gigabyte GA-965P-S3 Intel P965
: G.Skill 2x1GB DDR2-800
: Sapphire Radeon X1950Pro 512MB
: FSP Group (Fortron) 450W
: Western Digital 250GB SATA 3.0Gb/s
: Samsung 18X DVD+/-R w/ LightScribe
: Apevia X-Plorer Black
It's a little over budget when you include shipping, but trust me, it will be a killer gaming rig! Heck, better than mine.
If you absolutely need it under $900, we can definitely tweak it a little bit to meet that requirement.
How are you liking it so far?