With the size of video cards these days it's a really good idea to get a new case. Plus you can get a great case for under $100. The one HPLovecraft has linked is a great case. I have an Antec and it was easy to get everything installed inside it (although I did have to swap the 2 front fans with each other to get my video card in). Not only that, but the Antec probably has a lot more fans in it then your Dell one which is important for a gaming rig.
Do yourself a favor and pick up your parts from Memory Express and build it yourself. You can get them to mount the processor to the motherboard in store, for free, which only takes a few minutes and is probably the only difficult part. Just remember to remove the excess contacts from your case when you mount your motherboard. Memory Express is great for taking care of your problems too. If something doesn't work properly they'll replace it. They ended up swapping almost every component for my PC because they found issues with them.
You probably want at least 6 GB or Ram and 1 TB Hard Drive. Probably don't need to get the most expensive video card on the market. You can get a great card for around $300. By comparison I have a Radeon HD 4800 (which they don't even sell anymore) and I can run BF3 on max settings.
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