I have a dell Optplex 320 which is a lesser modle than yours but not by much. Mine has the P4 where yours is a duel core. If it is a mini tower then the best card you can hope for is a HD 6670 by AMD.
If it is a tower with these mesurements of L:16.5" x H: 15.5" x W: 7" Then the best card is the HD 6770 by AMD. PSU should be a 400w for both cards and the ram well it depends on if you have a 32 bit or a 64 bit OS.
If you have a 32 bit OS then the most ram you can use is 4 GB if it is the 64 bit OS then maybe up to 16 GB. If you have to buy a new PSU go for the 500w in these brands only don't buy a cheap one.
The brands are Corsair, Antec, Seasonic, XFX, Silver Stone, and some OCZ models. I will link you to a list of tier 1 is the best and tier 5 is the worst. The reason I said 500w is because you can find them cheaper than a 400w.
I am in the states so I will link you do the cards so you can get an idea of what they are and price if you know how to convert US to the Pound. I don even have that options on my Keyboard. I hope this helps a little and good luck.
I would honestly look into eBay if I were you for parts. I picked up a perfectly working 550W Antec PSU for $14.23 + shipping. As far as GPU's go, the GT 640 is a good bet, decent enough to play games, just don't expect too much with your system. Assuming you are running a 64bit OS 4GB is good amount of RAM for a budget buyer.
The 6670 is just a little bit better than the GT 640 and here is the link to the bench mark for both cards. The only thing the 640 is giving is the 2 GB of Vram which is good if you are running more that one monitor.
Or your resolution is above 1920 x 1200. The BF3 that is out works real good if you have 1.5 GB of Vram. But between both cards they are so close with the 6670 just a bit better.
That is why your cost shows the 6670 a few pounds more than the GT 640. GDDR5 is a better memory for the card than the DDR3. So it will render a bit better. If you didn't have that tower you could have gone with a lot better of a card.
If you ever buy a bigger tower then you could transfer eveything over to the new tower and go with a HD 6770 which is even better or the 7770. I will link the bench marks for all the cards good luck.
The 6750 is just as good as the GTS 450 and I have found a lot of them so you can choose which one you want. And yes 450 does a lot better than the 6670. You don't have to worry about the towers.
They are all able to fit any computer you have to transfer over to it. Both cards are good so it is a toss up between them both. Here is the bench for the 450 vs the 6750. Same rule applies the 6750 is the same as a 5750. Good luck.
Ok the PSU's will work. If it is a top mont PSU make sure the fan blows out the back and if you have a top that is vented on the case then the fan can blow out the top or the back.
Same thing for the bottom mount. You never want the PSU to blow into the case always out. All PSU's will work with any case. Just remember not to buy a cheap PSU brand and use the list above that I posted. Good luck.
Yes sorry about that I guess I didn't unstand the question. You will need one that has the same dimensions as your's is now. Don't worry about the screw pattern because all most every PSU has the same screw pattern.
In my opinion you should get a gaming Case put your MOBO into it get the bigger PSU and the bigger card. Then use it that way for a year in the mean time start saving for a new MOBO and CPU.
You should have enough money by then to get a better deal on all the new stuff. Have a gaming case you can add case fans to help keep air flow and keep things cool. I have 7 case fans.
I have 3 200mm intake fans. one on top one on the front and one on the side. I have 4 exhaust fans. One on top a 200mm one on the bottom 120mm and two out the back both 120mm push pull set up.
My rig stays very cool. My core temps never get over 34c along with my CPU. My GPU under load gets about 69c some times 70c it depends on what type of game I do. But all the fans keeps my rig cool and good air flow.
You can take all of the stuff out of your dell and put it all into a gaming case. A lot of people get the cooler master haf 912 mid-tower. But you don't have to get that one. There are so many cases out there.
If you do get one you will be one happy person. I know I am. I have 3 rigs one is the same as yours but it is a 320. The other Dell is the Inspiron 580 and it was the one just before my gaming rig.
I got tired of going through low end GPU's and over heating all the time. Anyway sorry for the long story. If you do get a gaming case it won't matter what type of PSU you would get.
The case I told you about is around $60.00 US and they do have cases a lot cheaper. You want a full or a mid tower not a small or mini. The bigger the better. Well let us know what you think ans as always good luck.