While any of these builds is perfectly find for current online usage, you probably want to consider the intended longevity of the PC.
I've recently been considering a similar build for my father, and came to the conclusion that future PC usage will rely heavily on multiple cores, and 64-bit capability. He will never need a racing fast computer, but when newer software comes out with multi-core usage and 64 bit code the computer will still be useful--and I will not have to buy him a new one.
In order to maximize your buying power you want to purchase PCs of a certain heft. To explain--the lower the price of the PC, the greater percentage of the cost represents parts like the mouse, metal case, and other basic components. With a really cheap PC, most of what you are paying for are the basic parts--not for its actual processor+etc. This type of computer does not last very long, and is a poor investment--you will have to pay for a new case, a new DVD drive, etc, and etc all over again in a year or two when you replace it.
In short, a $500 PC that works for 6-7 years is a vastly better investment than a $300 PC that works for 2-3.
You do not need a fast and expensive graphic card, but you
should get one. Graphics that are integrated into the motherboard are never very good--and sharing your ram with graphics is a large burden on your system. Anything with 512mb of dedicated graphics memory will boost your system a lot, and would be plenty for you.
I wouldnt drop below 3gb of ram. 4 preferred. My Vista computer is currently using 1.37gb to run firefox and some other stuff--and no matter what your computer usage you want a little room to grow.
More expensive options:
My first two suggestions, closer to the top of your price range--both have relatively powerful processors that
vastly outpower the Athlon™ II X2 240
While you wont take full advantage of these now, these quad core processors will age much better than the skimpy dual core, 32 bit systems you see in stores. Also, ZT computers are built in the US.
$569
ZT Affinity 7345Mi-37 Core 2 Quad Q9300(2.50GHz) 4GB DDR2 500GB NVIDIA GeForce 8400 GS Windows 7 Home Premium - Retail
$559
ZT Affinity 7343Ma Phenom II X4 925(2.8GHz) 4GB DDR2 500GB ATI Radeon HD 4350 Windows 7 Home Premium - Retail
Two cheaper options: These are the lowest I would go in power without starting to lose value as described above.
$419
Gateway SX2800-03 Core 2 Quad Q8200(2.33GHz) 4GB DDR3 640GB Intel GMA X4500 Windows Vista Home Premium 64-bit - Retail (this might come with a windows 7 upgrade coupon.. not sure) (Also, If I bought this for my father I would stick a $25 graphics card in it.)
Comparable to this computer here would be the
Dell Inspiron 546 that you have already found.
Were I configuring this computer for you or my father I would bump up the specs you chose a little. I would say your minimum should be: Windows® 7 Home Premium, 64bit, AMD Athlon™ II X3 425 (2.7GHz, 1.5MB) and 4 gb of ram. (the x3 is a triple core--a good bargain thats in between the dual core you picked and the quads I listed above)
This totals $458, or you can add the 4350 graphics card for an extra 60 bucks.
Best of luck! Let me know if you need an explanation of why different processors are better, or anything like that.