At the moment I am restricted to a laptop with 512MB of RAM, a 120GB HDD and a 1.5GHz Intel processor for all my development and general browsing needs, and as you can probably tell using it for anything modern is a painful experience. As a result I've decided to buy myself a new desktop computer, one that will stand the test of time and one that can be upgraded easily.

Rather than build the machine myself I've decided to go through Dell as I've had good experiences with them when purchasing computers for my family. I've had my eye on this as it's got a good amount of RAM, has a decent-rated processor and isn't priced too badly.

http://www1.euro.dell.com/uk/en/home/Desktops/inspiron-580/pd.aspx?refid=inspiron-580&s=dhs&cs=ukepp1&~oid=uk~en~20211~inspiron-580_d005827~~

Intel® Core™ i5 Processor 750 (2.66GHz, 8MB)
Genuine Windows® 7 Home Premium 64bit - English
Display Not Included
ATI Radeon™ HD 5450 1GB DDR3 graphics
6144MB Dual Channel DDR3 [3x2048] Memory
1TB (7200rpm) SATA Hard Drive
DVD +/- RW Drive (read/write CD & DVD) with DVD Burn software
1 year of coverage included with your PC
McAfee® Security Centre - 15 Month Protection - English 

After the pain of using a slow laptop for all this time the main thing I want is speed. I may look to play a couple of basic games on it, nothing too powerful. Obviously I'll be doing some development on it too so it'll have to be able to handle the latest IDE's and Database tools like SQL Server pretty quickly.

Finally, should I ever need to improve it I'd like to be able to add more RAM and change some of the parts. I wouldn't have thought this would be a problem but a few people I've spoken to have said that the amount of RAM the motherboard can handle isn't that great. Is this true? How long can I expect to be using this computer before it's too slow?

Thanks in advance for the help.