Just wanted to mention a couple things in reguards to the psu in the Dimension 8400. First, the stock psu is a 350 watt unit. It's listed in the 8400 spec sheet and printed on the sticker attached to the psu. Second, PC Power & Cooling offers direct replacement psu's for the 8400 and other Dells. I just put one in my 8400. It's a Silencer 500 watt (550 watt peak) psu. The recepticle for the power cord is in the correct location and there's no on/off switch to have to worry about making a hole for...just plug and play. I got mine at the end of January from newegg. They were on sale, plus an additional 20% off code and a $20 mail-in rebate. In the end, it cost me $50.
The only thing that was a pain was all the additional wires. There are connections for more components than the 8400 case could physically hold. There are enough connections to plug in 6 sata devices, 6 ide devices, two 6-pin graphics cards, an addition 8-pin connector, 4-pin cpu, main motherboard connector, floppy drive power connector, and I think I'm forgetting some. It really was a pain to find a place to route the wires in the 8400 case, but, it can be done cleanly without restricting air flow...it just took more thought than I had planned.