What parts should i change

Solution
i cant answer because most dell use custom pattern motherboard

all i can suggest is you try first with the new build

if cannot, buying new psu and case will not be a pain. $20 for psu and $30 for case
3rd gen ivy bridge i3 is better

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: Intel Core i3-3220 3.3GHz Dual-Core Processor ($120.58 @ Amazon)
Motherboard: ASRock B75M-DGS R2.0 Micro ATX LGA1155 Motherboard ($49.99 @ Amazon)
Memory: Patriot Viper 3 4GB (1 x 4GB) DDR3-1600 Memory ($23.99 @ Newegg)
Memory: Patriot Viper 3 4GB (1 x 4GB) DDR3-1600 Memory ($23.99 @ Newegg)
Storage: Seagate Barracuda 250GB 3.5" 7200RPM Internal Hard Drive ($51.62 @ Amazon)
Video Card: PowerColor Radeon HD 5450 1GB Video Card ($9.99 @ Newegg)
Total: $280.16
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-07-09 20:57 EDT-0400)