Dell power supplies are just rated as continuous units meaning they can run at their rated wattage and peak higher.  I would not necessarily say it would peak at 500 watts, however.  You can use this power supply calculator HERE to get an idea of what size power supply you need based on using the GTS 250.   Generally, it is best to add 10% - 20% to whatever number you get.  You can probably figure that under load or gaming that your entire system might need around 250 watts with a GTS 250.

Different power supplies can be more efficient at varying levels so, if for example, a certain power supply runs the most efficient at 50% of its maximum, then a 500 watt unit would be the right choice.  However, some may be more efficient at 60% or 70%.  The best thing I can say is to give it a try.  You are not going to damage anything. Obviously, if you have games crash on you or PC shutdowns occur, then you will know that the PSU is not up to it.  Since your system is older and your power supply has aged, it may behoove you just to replace it if you are going to put it under the stress of gaming on a regular basis.   I do recommend good brand name power supplies such as Antec, Corsair or Seasonic and not cheap ones just because they have more wattage.  Your current power supply would be better than a cheap one that has more wattage. Having said that, I have had an Antec 500W unit blow out on me in just one year.  Antec did replace it since it had a 5 year warranty with a much nicer newer model, but I am currently running a Corsair 650W unit and have the Antec sitting around as a back up for right now.  I have seen where others have run the HD 4870 on the 375W Dell unit and it uses more power than the GTS 250 so it is really up to you.  Also, you may want to verify that the dual slot cards will fit in your BTX case with that housing around the CPU heatsink.  

Another thing about some of the GTS 250's out today is if they have a variable fan as some do not.  You can tell by the wires coming off the fan.  If they have just one or two then it's not variable and runs at 100% all the time.  This HERE is the one I have even though I have had mine for over a year now and mine came with a higher core clock speed of 756mhz which was standard then on the EVGA versus 738mhz now.  

Thanks for that, but i was more leading to the fact of if my CPU can handle it, which is my main question.

pairing a nvidia 250 with an old pentium d is a waste of your money ... i would save your money and buy a new system or pull a cheap one out of the dell outlet center before trying to update a system that slow and outdated.

then what is the maximum graphics card my processor can handle then?

It is not that it cannot handle it, it just will not perform as good as it can with a better processor.   You will just experience lower frame rates in games compared to an i7, i5, Quad, or Core 2 Duo.  Cost should be your priority when upgrading an older system so you don't want to spend a whole lot.  What is the purpose for the upgrade?  Having to upgrade the power supply along with the video card would account for too much expense.  If you can settle for the 9800 GT, maybe you should consider it.  You could also take a look at the HD 4770 shown HERE as it uses less power than the GTS 250 or 9800 GT and performs right in between the two and sometimes as good as the GTS 250.  

so you suggest the ATI HD 4770 or the Nvidia 9800 GT??? and my specs (and possibly an upgraded PSU) will be able to handle it?

Do you have any dedicated video card right now?  If so, what do you have?  If your purpose is to play more current games and get some decent frame rates along with the appropriate resolution and game settings,  then either card should be fine.  Again, I would not upgrade the power supply unless it fails.  Also, like I said before, make sure any dual slot type card will fit your BTX style case.   This HERE is one of the most popular 9800 GT's at Newegg with a good rebate from a good manufacturer.  Just open up your case first and take some measurements so you will know it fits.  

i was just looking around on the net and i have a Dell 0x8582 motherboard with an Intel i945P chipset and a LGA 775 cpu slot. so i was wondering can a Intel E6500 work with my motherboard as it uses the LGA775 slot? and is the E6500 alot better than the 830?

Also to answer ur question i have an ATI Radeon x600 which is pretty shocking lol.

Also to answer ur question i have an ATI Radeon x600 which is pretty shocking lol.

Shocking indeed so just about anything would be better.  

The 9100 motherboard and BIOS does not support any Core 2 Duo processor like the E6500 so this is not an option even though you have a 775 socket.  If I am not mistaken, you have to have the 9200 system to support a C2D CPU.  

so at this point in time the best card my system can run is a nvidia 9800GT or ATI equivalent?