Questions on a couple of things...

I have this prebuilt. Its motherboard comes with a PCI-Ex16. It has a 250W PSU with 12v@15A, E1400 (2x2ghz Celeron CPU [JUNK!]). Mobo is socket 775.

Questions.

Will the PCI-E (1.0) x16 bottleneck higher-end cards such as the 4850?

Can I fit processors such as the E8400 and the Q9550 (It's a bad prebuild, so I'm a bit wary..)?

As the case is smaller in size, is thick, and has 50/50 ventilation, what kind of cooling should I get?

I was told I could get a 4650 and an E5300 (gpu/cpu) without needing a PSU upgrade. But with an upgrade...yeah. The thing is, if a 1.0 PCI-E slot will bottleneck a higher-end graphics card, then there wouldn't be any reason to get another PSU.

Thanks!
Steve.
 
You would definitely need a bigger PSU for a 4850.

Obviously, the chips you list are 775. However, the replacement chip must be supported by the BIOS. The only way to check that is through the manufacturer website or by contacting them.

PCI-E 1.0 16x will not bottleneck a 4850.
 
What about processor though? I'd rather get away with upgrading the processor without anything drastic like flashing the BIOS. After doing some research, I found that this is compatible with all C2D procesors...is this likely to be true?

Using an 18'' monitor, I'm gaming at 1280x1024. Will a 4850 max Tf2/Cs:S? (16xAA/AF/Vhigh settings?)

Lastly, I have a small 15'' monitor (LCD). Is it possible to browse the internet and game at the same time? And will that slow my game down by a noticeable amount?
 
A processor upgrade is a good idea for more reasons than just gaming. Ordinarily I would say an E5200 and overclock it but I don't think your bios allows for such. For a few dollars more there's the slightly faster E5300 which is probably your best choice;
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116074
or for $13 more you can get a E6300 with .2 more ghz;
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116091
but that's a price increase of 18.6% for a speed increase of 7.7% so I probably wouldn't bother but it's your call.