I'm running the system in my sig on a 300W Antec power supply with no stability problems. It has a higher load than your upgrade plans.
The key is if your 300W power supply is a good one or a cheap generic.
Check the label for:
Name of the power supply manufacturer
+5V 25A or higher
+3.3V 16A or higher
Post the manufacturer and ratings and we may be able to give you a clearer answer.
And NEVER pick up a power supply "real cheap". With power supplies you get what you pay for and if the power supply craps out you can loose your entire system. The cheap power supplies can even catch on fire. It's better to get a 350-380W quality supply than a cheap 450W.
Quality names - Enlight, Antec, there are others too.
Check in the bios under hardware monitor, advanced features, etc. Or download a copy of sandra from sisoft. There's a shareware version.
And next time the case is open check the 3.3 and 5V current ratings on the power supply. Like I've mentioned in other posts. It's not the total power capacity on the supply anymore it's how much current it can deliver ta 3.3V and +5V. All the other voltages (+/12V, -5V, etc) aren't used much anymore.
15A @ 3.3
22A @ 5V
2A @ 12Vdc (for fans)
should meet the needs of mike09's system and thats less than 200W. If Dell knows exactly what it's going to put in the system it can skimp on the power types it won't need.
Ep, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS