So generally the point to buying most brand name/factory built machines is the fact that you get a warranty for the whole machine and not just the individual component makers like you would if you built it yourself, additionally you would get support or lack there of depending on who you purchase it from.
Now if you just want something fast and cheap and really don't care about warranty/support the cheapest and fastest option is to build it yourself (or have someone build it for you).
If you are concerned with warranty and are not that computer savvy I would seriously consider buying a brand name machine, but keep in mind a majority of manufacturers will be making quite a hefty margin off you (resellers allot less).
I think the machine the good Dr. linked first in the previous post is a pretty decent value provided you can find out whether or not they have good support.
I also came across a very good deal on a laptop that is a very good deal
http://www.newegg.com/Product/Product.aspx?Item=N82E16834115546I just purchased a lesser version of the same laptop with not nearly as good a graphics card for a second work station at my job and it is beautiful.
Despite what some may think Dell does has very good support for their products and I would configure the following machine with 4gb memory:
http://configure.us.dell.com/dellstore/config.aspx?oc=dddwqa4&c=us&l=en&s=dhs&cs=19&kc=productdetails~desktop-inspiron-537 For I think it was $734 you can get a pretty decent machine with actual support, the bigger monitor sizes and memory card reader was not much more money either, I would play around a little with it to get exactly what you want though.
Now if you want to build it keep in mind that you are pretty much your only support, many parts manufactures are TERRIBLE when it comes to dealing with any issues relating to a PC you built yourself and you are going to be responsible for any part that fails, all the trouble shooting involved that comes with it.
Some times a build can go well and everything works just like you would expect, but other times you may end up with a giant headache because there is some minor detail you miss or a setting you forgot etc.
Of course you have us to help you with a majority of any issues that may arise so don't let me discourage you into not building one it is a very cost effective option if you want the most bang for the buck.
Components you will need are as follows:
Case (Antec is a decent brand, but again anything is good in as long as it is ATX)
Case fans
Processor (Intel is a safe bet, but AMD is also a good alternative)
Processor cooling fan if applicable ( if you buy an OEM cpu you will need this; retail includes one)
Power supply (For the sake of future proofing I think a 600 watt or above is better bet)
Memory (at least 3GB if you are going with a 32-bit OS, a minimum of 4GB for a 64-bit)
Motherboard
Video card (512MB or higher of memory)
Hard drive (whatever floats your boat)
Dvd combo drive (will play dvd media/data discs and burn on to CD's)
Optional:
K1 Killer NIC (My fps went up and latency went down after I figured out how to configure it)
Speakers
Monitor
Keyboard/Mice
All of the optional stuff is of course all up to you and is usually not included in a factory built setup.
Software:
I recommend AVG antivirus
An OEM copy of windows Vista Home premium is probably the way to go if you want to avoid tons of updating and fucking around with installing the seven year old windows XP.
Now to the fun part I will go through, research, and link what I think you'd be happy with.
For a tight budget
AMD
Case
Antec three hundred 54.95 +S&H
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129042Case fans
Rosewill 120mm case fans 4.49 +S&H
http://www.newegg.com/Product/Product.aspx?Item=N82E16835200048Processor
AMD Athlon II X3 720 2.8Ghz 139.00 Free shipping
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103649&Tpk=720%20X3Power supply
Corsair 650Watt 99.99 Free shipping
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139005 Memory
Corsair DDR2 XMS 65.99 Free shipping
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145176Motherboard
Gigabyte GA-MA790X-UD4P 109.99 +S&H
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128387Video card
Radeon HD 4850 1GB 134.99 Free shipping
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150351Hard drive
Western Digital special edition 640GB 59.99 Free shipping
Dvd combo drive
Samsung SATA Dvd burner 23.99 +S&H
http://www.newegg.com/Product/Product.aspx?Item=N82E16827151187 Depending on which Os you want to use it can cost from 90-140
Total=697.87 +S&H charges + cost of an OS (another 90-140)
That is pretty much the cheapest I can find actually so anything else would be a bit more, and while yes they are all solid brand name parts I really don't recommend buying anything cheaper because allot of the cheaper brands are pretty much junk.
I can honestly say after going very thoroughly through allot of whats available that it may be more cost effective just to go with that Dell and if you want a better video card at a later point just upgrade it.