Welcome to the forums.
Regarding your first question, you should be able to get medium graphics settings out of that video card at least, with some settings up in the high range. You will need to stay away from HD textures, though - you don't have enough VRAM on your card to run those. Your CPU is old but is okay for the vanilla game. You'd need to be careful about adding mods to your game, though, especially ones that add a lot of new NPCs (who have lots of scripts attached to them and increase the demand on the CPU).
A complete playthrough, if you do the main quests and a good number of the guild quests and other side quests, will probably take well over 100 hours total to complete - some people can put many hundreds of hours on a single character. But you can definitely play it at your own pace and still make good progress. There's no quest in the game that has a time limit to complete. Some quests can be done in just a few minutes, while others may take a couple of hours or more to complete. But even in the longer ones, there are usually points in the middle where you can save the game, quit and then come back to finish when you have some more time.
You can save your game progress at any time (no checkpoints), and if you do get it, you should quickly get into the habit of saving frequently, and keeping multiple recent save files handy. Bugs and glitches do happen, unfortunately, and probably the biggest reason why people ragequit the game is relying on one save file, having the game glitch out on that file, and losing hours and hours of progress.