Well glad you will be able to use your card on the fix. I would not go to 32bit system. Everything is going be going to a 64bit and all 32bit programs should run on a 64bit system.
If you go to 32bit you will only be able to have 4 gigs of RAM. With a 64 bit you can use a lot more. Your third question don't worry about it. Your last question I don't speak UBUNTU. What country does that come from?
With your dell system you don't have much room to grow. Now a days you need 4 cores or more. With 2 cores you are limited to how big a GPU you can get without causing bottlenecking.
Plus there is your PSU will come into play on what type of GPU you can run. For now keep to 64bit OS upgrade your ram to 6 or 8 gigs. Go ahead upgrade your HDD to 1 TB or more than 250 gigs.
Then I would start saving money and build you a nice system. This system is a store bought PC and 9 out of 10 are not ment to be a gaming PC. I have gone through 4 store bought PC's.
I would go through one a year. So instead of spending a $1000.00 a year I desided to spend an extra $600.00 and build one. Back at the movie you said jumped I would check your settings in your CCC.
Make sure your PC is dust free on the inside and that you don't have a lot of heat build up. Make sure all your cables are tight and make sure your card is fully seated.
I will link you to a program to monitor your temps and set your fan on the GPU to speed up as temps rise and also watch your FPS. All the links are to monitor your temps from CPU to core and GPU.
http://event.msi.com/vga/afterburner/download.htm
http://event.msi.com/vga/afterburner/images/Afterburner%20User%20Manual.pdf
http://www.techpowerup.com/gpuz/
http://www.techpowerup.com/realtemp/
http://www.alcpu.com/CoreTemp/