Why is it that it would be such a bad idea to upgrade? I'm not trying to debate the fact that it's a bad idea, I'm just curious as to why.
New tech uses PCI-Express. Your motherboard has AGP, a dead interface.
New tech uses DDR3. Your motherboard has DDR.
New tech uses AM3 or LGA 1366 or LGA 1156. Your motherboard has socket 775 or 478.
Let's say you upgrade your graphics. You'll have to get an AGP card, which will be pretty much completely overpowered (since nothing is really that slow anymore) and will be bottlenecked by your ram/cpu. Not to mention, you'll have to toss it should you ever upgrade your motherboard.
Let's say you upgrade your processor. The best you can get on a socket 478 is a Pentium 4 Prescott core, which only comes in single core flavors. Not to mention, it's impossible to find, and is the rare case where AMD absolutely owns intel in every way shape and form. Seriously, P4 sucks.
Let's say you upgrade your motherboard. You'll have to get a new CPU, RAM, and Graphics card. Pure and simple. That IS the computer. Anything else can be re-used, including the terribly slow pata hard drive and cd rom drive.
If you want to upgrade for cheap, get an amd platform with a decent µATX motherboard. If you get a 785G chipset, you can pass on the graphics--the onboard 4200 is technically equivalent to the 9800 pro, and should be able to run SC2 awfully slowly on a low resolution. If you get a phenom II x2/3 or an athlon II x3, you can unlock those to a quad core with a good chance, so long as you have a 710 or 770 southbridge. The beauty of quad core is how well it handles multithreaded tasks. If the task is not multithreaded, it allows you to multitask. My dual core slows down firefox if I'm running one core at 100%. Not so with a quad core.
The am3 socket will also allow you to buy a 6 core phenom way down the line if you want, something the competing LGA 1366 can't offer. The 1156 might eventually support 6 cores like the 1366, but the 1366 will never support anything lower than a quad probably. Granted, the 1156 has the i5 750, which is more than enough processor for anyone, since it performs much better than the i7 920 for the price.
More than likely, you'll want a new power supply too. That's why you should put an amount on your upgrade, or just say "the cheapest you can make it with the ability to upgrade later", and we can find you the best fit.
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