Recommend me the best Dell for gaming in the price of $400-$900.

My take here, you should be able to get a year or two more out of your current rig simply by swapping out the power supply and adding the vid card - I think a much more economical way to go.

Yeah, but say I get a 600w or something right, then get a GTX 560 Ti, wouldn't that Dual Core Bottleneck it?

Also, I might get into recording too, which a better CPU will help with.

I don't think you'll be bottlenecked as much as you will be improved on the current rig. Again, the price difference of upgrade vs new makes me consider that as viable.

Is there a reason you want an OEM machine and do not want to DIY? IMO, it's both fun and informative - just curious.

Lots of things, time, inexperience, overall my lack of understanding about how computers are built.

But yeah, the main reason I'm trying to update to a quad core or even a core i3 or core i5, is mainly so I don't get as many lag spikes in games. Mainly CPU intensive one's like BF3 and Blacklight: Retribution.

I also wanted one with a PSU around 460-500w, so I could upgrade without having to change the PSU.

Okay, here's what I got for you in the Dell listings. The XPS8500 here
http://configure.us.dell.com/dellstore/config.aspx?oc=d...
It comes with an i7 Ivy Bridge CPU, a graphics card already (so I will assume at least a 450W PSU - more on that in a moment), 8GB of ram, 1TB HDD, Win 7. Looks to be decent buy from Dell.
Couple of things to note; 1) There are four models of the XPS 8500, I chose the least expensive that had an i7 processor. 2) PSU info, finding PSU info from Dell seems to be harder than anything else. I'm assuming a 450W but it may be as low as 300W since the included video card seems to draw about 70W. Finding info about that video card is a little difficult, I did find some but I am unsure of the accuracy since the card is an OEM card (can't be found in stores).
If you start to configure the XPS 8500 from that link, a pop-up will ask if you want to chat with a Dell rep - they might be able to provide the answers about their PSU's (I'd actually be surprised if they can though)
As far as the DIY way, other than the time factor - you would want to dedicate about 6 hours for a first build - this video (only if it might seem interesting) explains what is involved in building your own computer
http://www.youtube.com/watch?v=d_56kyib-Ls&feature=relm...
Again, only if you might be interested, not quite as hard as most people think. BTW, that's the last I'll bring it up, just for consideration.

Well alright, when I save up about $900 I'll probably get that then.

However, if I could just replace the PSU and get a GTX 560 Ti for less, I may do that.

See the thing with PSU replacement is, is that I don't want to end up breaking anything in the computer in the process.

So, I'll only do the new PSU + GTX 560 Ti combo, IF there's like a detailed step-by-step video on it. Basically, a "How-to-replace-a-psu-for-dummies" video lol.

I basically just want to have less lag spikes in badly optimized games basically.

This video seems to be okay in showing how to replace a PSU, there may be others that are better.
http://www.youtube.com/watch?v=UzolRkN7Eew
and then this one (by the same people) explains how to replace (but installing a new one you can start at 2:20 in the video) a video card.
http://www.youtube.com/watch?v=5YRlO-55njI&feature=relm...
Again, quality is okay but there may be better. I kinda like the little comments that come up during the vid.
If any questions, ask.

Replacing or even building a PC is fun and exciting. Just make sure your not statically charged, touching a metal case or table would work to verify you don't have any static charge.

The Intels i processors are great for video editing. I would upgrade the graphics and PSU first to see how well your PC would handle, both the video card and the PSU could be saved for another build or sold separately. You can also sell the system as is but thats up to you. If your not even sure if you can even sell the system to anyone i would try to find a buyer before getting another PC as its easier to buy a new PC but it's harder to sell your old ones.

There are a few ideas if you haven't tried them already, shut down any programs at startup, cleaning your hard drive, defragmenting and checking to make sure your current system is at optimal performance. That may or may not improve your gaming performance but it helps to have a PC that doesn't have anything slowing it down any. It doesn't hurt to make sure it's not just your graphics card that is causing your game spikes. The easist way of doing this is to have the Task Manager open, use the slow speed then run the game allowing it to spike then close the program down and see if your CPU is sticking at 100 percent. Solid 100% usage means your CPU is capped off and most likely causing the game spikes to occur. If it looks like the usage is random which may hit 100% but it doesn't stay solid most likely its something else causing the game to spike.

I think its your video card though since i would imagine that there would be freezing involved and according to the BF3 System Requirements for processor power you meet or exceed it although i thought quad cores didn't affect gaming performance.. i could be wrong but it sounds like to me a GPU upgrade is in need or if its only spiking when the game is being loaded, woudln't hurt to monitor your hard drive or cd/dvd drive to see if the spikes occur when the cd/dvd/hard drive is under load.