I'm looking at building my own gaming computer so I've chosen some parts but I have looked on the Dell website for the same and have found one that is similar but I'm rather new to choosing parts so could someone tell me the difference between the two. (and if there is anything to improve on either,I would be grateful to know)
-My custom build:
Asus P7P55D-E Pro Intel P55 (Socket 1156) DDR3 PCI-Express Motherboard
Frankly, neither is that good. Your CPU is bad, but your GPU is good. The Dell's CPU is better, but the GPU is weaker. The Dell will have low quality parts, while your build doesn't. The Dell includes Windows, but your build doesn't. Finally, you have two cases listed, bother of which are tiny.
I would highly recommend following the guidelines (the link is in my signature). I know for a fact that I can throw something together that will beat either build, but I need to have a budget, what you're using it for and some other information.
You failed to mention your budget. If you plan be have a "gaming rig" then instead of dell look into "cyberpower"
As listed your system's specs aren't too impressive: however that doesnt mean that you could not use parts at that level to play games. Remember that some of the newer games are very demanding on hardware if you want to run at high quality settings.
As madadmiral said: follow the "asking for new build advice" guidelines. it lets us posters give you more specific advice.
Here's a list of some suitable alternatives if you can't find the exact part:
RAM: Any non-OCZ sticks with 1600 mhz CAS Latency 7
GPU: None. Possibly the GTX 480 if you want to pay more and get less. If you go that route, you'll need a 1000W PSU to handle SLI.
HDD: Seagate 7200.12 1 TB
Case: HAF 922, Coolermaster Cosmos, Antec 902, Antec 1200
PSU: Same wattage from Corsair, Silverstone, SeaSonic or a different model from Antec
Optical: Whatever is cheapest
HSF: Sycthe Mugen 2 Rev. B
Possible changes if over budget (in order of preference in terms of performance):
Leave out the HSF until you decide to overclock
Drop the HDD to the 500 GB model (likely only $20 or so saved)
Switch from the i5-750 to AMD's X4 955 and Gigabyte GA-790XTA-UD4 (around $100 saved)
Drop the 5870 to a 5850 (around $100 saved)