as far as just the computer itself (not counting internet) I think it would vary depending on the computer and what components you use, and the setup. With my computer, Its the hard drives-Im using a RAID 1 array, which actually slows down hard drive speed (the reason for it is the redundancy, everythings backed up in case one drive fails) With the CPU being the next biggest bottleneck. With most store bought computers, often its the video card in desktops, and the CPU in laptops. Bus speed can be a bottleneck, especially with intel based CPUs (sorry intel fans, but your bus speed is rather slow though its better with the i series) While AMD CPUs have a much faster bus, so that isnt their bottleneck, but they arnt quite as fast either. . . so you gotta weigh the pros and cons and figure out exactly what you want to build the computer for.
Here are some things to focus on, on any computer, never skimp on the motherboard or PSU. they are the backbone, you want good quality, with lots of room to upgrade in the future. You can skimp on the other components and upgrade them later, but dont skimp on those 2.
If your into graphics intensive games like crysis or say Battlefield bad company 2, a decent 2.8+ ghz quad core, 4 gigs of ram, and high end video card(s) (dual GPU setup is very handy) are probably what your going to want to focus on. Especially the video cards, 4 gigs of ram is a must but you dont need to go to extravagent, a decent quad, AMD or intel, but definetly buy at least one high end video card if not to. If your more into games like WOW, a high end quad core, and at least 4 gigs of really fast memory, are probably more important, because they are more CPU intensive instead of graphics intensive. You will still likely need a decent vid card, but prolly only in the higher midrange cards.
If you just want a HTPC or home computer, dual core CPU, 4 gigs of ram and mid range video card should do you fine.
If your looking to do a lot of storage, like server, turning it into a DVR, etc. spend more money on the hard drives. with gaming and movie watching, a single 250 to 500 gig HD should be fine, but if your going to be storing and accessing a lot of data on your hard drive, getting multiple hard drives and putting them into RAID 0 or other raid formats that include striping-will greatly improve your search time. Spending the extra money on the 10,000 RPM or SSD drives would be a good option then to.
Primary system: Motherboard: ASUS M4A89GTD PRO/USB3, Processor: AMD Phenom II x4 945, Memory: 8 gigs of Patriot G2 DDR3 1600, Video: ASUS ATI 4890 and a Saphire 4890 in Crossfire, Storage: 1 WD 500 gig HD, 1 Hitachi 500 gig HD, and Power supply: Coolermaster 750 watt, OS: Windows 7 ultimate 64 bit.
Media Center: Motherboard: Gigabyte mp61p-S3, Processor: AMD Athlon 64 x2 6000+, Memory: 6 gigs Patriot DDR2 800, Video: Saphire 4850, Storage: 500 gig Hitachi, PSU: OCZ Fatal1ty 550 watt modular PSU, OS: Windows 7 Ultimate.
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