Which desktop should I get?

Hi, I am new to this forum so I hope that this is in the right place. Okay, so I am looking for a new desktop to replace my old emachines celeron based computer. At first, I was thinking of building a computer for the experience but since my budget is only about $500-$600 and I don't have an operating system, I think that it would be better to buy it pre-built. This computer is mostly used for productivity with maybe some games, but nothing like crysis or any other advanced games. The main thing I want with this computer is it to be fast as that is my biggest issue with my current system. So, I was looking at two computers in particular, but I have no idea if I am even going down the right path. The first computer I was looking at was a dell inspiron 530 with a q6600 processor, 4gigs of ram, and integrated graphics (I would hopefully upgrade to a low end graphics card later because I heard that it is best to have a graphics card to run Vista). Originally, I could get this for $500 and maybe $50 for a descent graphics card, however, the sale just ended and now it is about $580 without the graphics card.

The other system I was looking at was an iBUYPOWER Gamer Power 900X that was only $500 off newegg and came with an 8400 GS graphics card which I think is a descent graphics card (remember I'm not doing any hardcore gaming). Also, it has a q6600 processor and 4 gigs of ram. Here is a link http://www.newegg.com/Product/Prod [...] 6883227121 . The downside to this desktop is all the negative reviews, i have never heard of ibuypower, and according to some reviewers, it is very loud.

Also, briefly worth mentioning, I found some desktop called a zt that had similar specs to these two computers and for $500, although it doesn't come with a graphics card and the psu is only 300w so it would be hard to find a graphics card that would work with such little power.

So, those are the three desktops that I have found however I am not that tech savvy and I don't even know if these are good deals so please help me out. If you can recommend anything better, please do. Thnx in advance.