So I want to upgrade my really old prebuilt PC (HP a1740n).
1.86 GHz Intel Core 2 Duo E6300
2 GB ram
Seagate ST31000528AS Barracuda 7200.12 Hard Drive - 1TB, 7200 rpm
Sapphire Radeon HD4550 512 MB DDR3 VGA/DVI/HDMI PCI-Express Video Card 100252HDMI
DYNAPOWER USA ADW-500 500W ATX12V Power Supply
I have a budget of around 500 dollars. I was just thinking of getting an Intel Core i5-2500K and a corresponding mobo. I don't really know alot about the parts but I'm mainly trying to multitask (i.e. play a game, pause, web browse, seamlessly). I don't really think my graphics card is up to the task of running modern games. Ideally I would like the parts to run SC2 at 1650 by 1050 on max settings.
Well I'd say it depends on how important a 2500k is to you.
Because if you do get a 2500k ($200), you'll need a [decent] mobo ($100-150), and ram to go with it (2x2gb 1333/1600 for $50). So that takes you down to about $100-150 left. You could get a pretty good graphics card for that money like a 6790 or a 550Ti.
But if you get an AMD phenom x4 BE, you would be getting an extra $60-90 more for a card like a 6870 or a 6950 1GB which overall would give you much better quality gameplay in then you would be getting with the 6790 or 550Ti.
You may need a new case, and in my opinion you should get one if anything for better airflow. But if the case works it would save you some cash. Just make sure it is compatible with whatever board you get (ATX or mini-ATX are the most common)
I'd go with the 2500k if at all possible. They run circles around the cheaper AMD chips so you'll get many more years out of the 2500k than an AMD system. Getting it all within budget just may be a bit difficult though, but as long as you recycle what you can and perhaps push your budget to make room for a good GPU it can happen