You will definitely have problems, the HD 3450 and the build is just a tight fit. Video card manufactures often overestimate the power requirement because people often cheap out and buy terrible power supplies (that do not deliver the labelled watts). If you do place the HD 4670 then your computer will probably not turn on due to insufficient wattage. Might I recommend going for a power supply upgrade?
If you can afford it, this power supply will do nicely with extra wattage to support future upgrades, and this one actually delivers the wattage labelled
compared to your average cooler master or thermaltake power supply. http://www.newegg.com/Product/Product.aspx?Item=N82E16817371045&name=Power-Supplies
Edit: If you want specific wattage please list your exact computer specs, the manufacturer page on Dell shows your computer has the possibility to come with several pentium 4 processors. (Thinks its time for an upgrade
if your planning on real gaming )