Since you are looking for a desktop you seem to be in luck for that range. Of course remember pricing and such constantly changes, but you posted this earlier today(well, really early this morning in my time zone), I can give you an up to date price.
I don't know if you have a brand preference, so I guess I'll just give you the best results, but I'll note my opinion as well. This really is an opinion topic anyways, since you used the word 'best'.
There is only one thing I must note. Desktops with wireless internet are not that common still. Yes, there are a good bit out there with that built in, but in most cases, a desktop computer will not have wireless internet pre-installed. But luckily, with your price range you can buy a wireless adapter for $50 or less and still be under budget.
http://www.bestbuy.com/site/olspage.jsp?skuId=999988700050004&type=product&id=pcmprd88800050004
This one is a Dell with 3 GB RAM and a 500 GB Hard Drive. You get a 19" widescreen lcd moniter. The only thing that might be a concern would be the graphics card on this PC. Most recent graphics cards can be configured so you can have higher video memory, but i can't say for this one.
http://www.bestbuy.com/site/olspage.jsp?skuId=999990400050003&type=product&id=pcmprd90500050003
If you are not a brand freak, then this is a good PC. At under $600, this PC has 2GB RAM and a 320 GB Hard Drive(which are both average I guess), but it has an intergrated nVidia graphics card. This will put you in a decent place for gaming. Not top of the line but probably good enough to play most games. This also has a 19" widescreen lcd.
Hmmm.... well, I might have to make another post later. Looking for good PC packages online is not easy. The best way is in the store or through the sale ad. I can't seem to find our sale ad. I'll get back on later and probably use the online sale ad. It is the best place to start looking for good PC packages.
Edit:
Hmmm... well I actually didn't see anything good in the ads. The first computer I listed(the Dell) was there however. It actually is not a bad deal and I believe that was just under $700(so a bit more after tax).
I did find this deal on a wireless USB adapter though.
http://www.bestbuy.com/site/olspage.jsp?skuId=8305602&type=product&id=1173577562601
This is a wireless N adapter. I don't know what kind of router you will be using, but I don't see it being much faster than this adapter. 300 Mbits /sec for $50 is a steal actually. Of course after taxes and all, the Dell PC with this adapter would actually put you a bit over $750. But All in all, it is a great bundle. I realize you were not looking for a printer, but in most cases when you get a moniter with a PC, you usually are bundled a printer as well. And simply buying a moniter and PC seperately will cost you a good bit more. But it isn't bad since you get a 3 in 1 print/copy/scanner here.
I wish I could have been a bit more help here but it seems this week isn't the best week for Best Buy computer deals...
Just as a side note. I am not a big Dell fan. Dell and Gateway are probably my least favorite PCs. I've owned and liked eMachines, Acer, HP and Compaq. Acer and HP are probably my favorite brands. Also, I'm not a bit Intel fan. But the reason isn't because I simply like AMD better, it is because AMD has 64 bit processors. and you don't need to be a guru to know 64 bit > 32 bit. So when I look for computers I try to find an AMD 64 processor. My last 3 computers have had them. I just think they work faster.
Also, that second eMachines only had 2GB of RAM but for $29-$34 you can buy 1GB of RAM from Best Buy. So keep that in mind since the eMachines had a better graphics card. I think the eMachines PC with another GB of RAM and that Wireless N USB Adapter is a great deal and should leave you happy. This is just my opinion. If you can wait a week PM me and I'll look at the new deals to see what you might be able to get.