I was looking into low-profile graphic cards for a Dell Dimension 521. I have found quite a few but noticed some say 32bit 64bit and 128bit.Would my computer be able to run those, and whats the difference?
Is that your PC?
Well, it only has PCIE slot 1.0 and 305W PSU. Not sure about CPU, which one your CPU? AMD Athlon™ 64 X2 dual-core? AMD Athlon 64? or AMD Sempron?
No offense, but even AMD 64 X2 is a little bit weak for today's standard.
Right now you're using onboard video, right? What are you going to do? Just play music and browsing or playing games as well?
HD5450, GT210 and HD6450 is not a gaming card, so if you want to do some gaming then you can take HD5570, BUT, ALL HD5570's are 2.1 cards. Many issues running 2.1 cards on 1.0 slot like yours.
Your choice is a very little, buddy. So if you want to gaming then you'll better sell your PC and BUILD your own gaming PC...
But if your purpose is just for HTPC, then GT210 is a good card for that goal.
You probably are better off looking at a new build at this point if you want good gaming performance. Your PSU will support up to an HD 5670, which is an okay gaming card, it will do well as long as you don't want high settings at 1080p. However, I don't think there are any low profile versions of the 5670 in stock anymore at any major retailer, you'd probably have to try ebay or something like that.
The 5570 is the best card you can get that still has a lot of low profile versions still floating around. You may run into the PCI-E 2.1 compatibility problem. It is hard to say for sure, some 1.0 boards have had problems, a lot of other 1.0 boards don't. You'd have to roll the dice and see what happens if you want to upgrade your system.