You'll definitely want the new quad core AMD processor over the Intel Dual Core. That's an old CPU, it's not even a Core 2 Duo, which itself is a generation old behind the new Core iX series. You won't need any more than 4GB for playing Oblivion or anything else, unless you plan on doing video editing or running a ridiculous number of applications at once. But the Athlon II X4 can handle faster memory than 800MHz -- buy the fastest you can. The price difference isn't that much but the performance increase will be worth it.
2x4GB is a better configuration if you want to have room to expand your RAM in the future. Most motherboards have four RAM slots, so putting two 4GB stick in would leave the other two slots empty for future expansion. But 8GB is way more than enough for anything a regular gamer and user will ever use, so I'd save money if you're going to put 8GB in and go with the 4x2GB configuration.
Either video card is fairly weak. If it were me, I'd get the AMD chip, put 4GB in a 2x2GB configuration at 1066 or 1333 MHz, depending on what motherboard you choose, and I'd wait to buy the system until I could afford a decent video card. It's probably about twice the cost of a cheap GT 220, but Newegg has a Radeon 4870 card for $100 after a $30 rebate:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150436&cm_re=radeon_4870-_-14-150-436-_-Product
That's the card I'm recommending right now to people looking to build a decent gaming rig at a cheaper price point. It was the flaghsip single-GPU solution of their 4-series of offerings, it blows the GT 220 out of the water and is faster than ATI's newer, more expensive Radeon 5770 and 5830 cards. If you can afford the extra money for the 4870 you will be much, much happier playing Oblivion or any other game. The GT 220 and Radeon 5450 are rather anemic.