I just filled a huge page with my thoughts on Dell PC's, but you are going to buy one anyway so......
The difference between the two systems is basically the chipset, the 8400 uses the Intel® 925X Express one which supports PCIe 1x and PCIe x16 for graphics cards.
The PCIe x1 slot gets blocked off by the fan on a decent graphics card installed into the PCIe x16 slot, and if you use a lame graphics card in the x16 slot, it's fan will be blocked by any PCIe x1 device installed into it's respective slot......... Bad motherboard design.
The 4600 uses the I865G chipset which is Onboard Graphics (a definite no no) with an AGP slot to add a Graphics card.
I'm sorry..... I was trying to make some positive points here, but I've already wasted too much of my time trying to explain away Dell PC's, and that violent mood swing is starting to kick in.......
Good Luck with your new PC.
Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24" WS LCD