Depends greatly on if your optiplex has a standard psu or not (sorry, too lazy to search it)
Regular pc: Thermaltake430W goes for cheep... I get it for 38$
Con is that it is 2x 80mm Fans front and back... therefor it does not really aid the cpu air circulation like the 120mm fans do.
Recommended for pc's that already have fair ventilation and does not have a sick videocard.
about 75 to 80$ but worth every penny. No Flaw in my opinion.
I have used quite a few AcePower PSU's...
Cheep Powersupplies but have not failed on me yet. (Sold about 8 computers with them and at a previous job seen over 30 DVR Server using them... none have failed but 1 520W on a pc I built to a friend that is like a pc curse...(failior due to light switch that also turns off the outlet))
They all have 120mm fans, solid casing... look great but I think the inside is fairly cheeper.
Antec is pretty fair but I strongly recommend a 500W Thermaltake.
For bigger power requirements I lack the experience so the only PSU I can recommend is the same that I am using.
600W OCZ Stelthsomething... After googling it I saw good reviews and so far its been great.
Not a fan of OCZ but the PSU did a great job for fairly cheep.
My 600W is running a phenom9950 + 260GTX + 2 320gigs on an older Nforce570 chipset... not a very green pc.