Laptop battery has died, and general laptop upgrading questions

[Solid Snake]

Despite my intense affections for PC gaming, I am by no means whatsoever a proficient techie. I've never even upgraded the hardware of a computer I happen to own.
...and now I'd like to upgrade the hardware of a computer I happen to own.

More specifically, the battery of my laptop has apparently died and needs to be replaced. Unfortunately, I don't know exactly what 'part' or 'component' of my battery has died. All I really know is that a red light is constantly flashing on my laptop and it now only works when it's plugged in, thus relegating it to essentially a mobile desktop.

While I started thinking about a replacement, though, my tax returns suggest I'll be getting a few hundred more dollars of sweet cash I wasn't anticipating.

So I'm now debating whether to upgrade my current video card (I presently own an Nvidia GeForce 8600 M GT, which is solid enough to run Mass Effect 2 with only occasional hiccups but falls well below recommended settings.)

The biggest problem I had with Mass Effect 2 was, strangely enough, sound: occasionally the sound would...well, something strange would happen, and the sound would keep playing, but there'd be a metallic resonance or tone. Hard to describe. Possibly something that a new video card wouldn't even fix, though I have confirmed no scratches or anything of that nature on the CD, and I don't have this problem with any other game. I'm just glad ME2 had subtitles.

I'm also debating whether to upgrade my current hard drive. My hard drive presently is 142 GB but I only have 23 GB free. I'm starting to notice a slight slowdown which I'm hoping isn't being caused by my hard drive overload.
My hard drive really isn't a priority, though: I can delete stuff to make room for the new games I may (or may not) buy. It just may become difficult once Starcraft 2, Diablo 3 and Mass Effect 3 all come out...but those are the only three new computer games I intend to purchase (unless Portal 2 is announced.)

As for the rest of my computer's specs: I own a Dell Inspiron 1520, it's a bit more than two years old. Nothing aside from the batteries are broken, so any upgrades I'd perform would be purely because I had the extra cash and wanted to improve performance. I have an Intel Core 2 Duo CPU -- well, two of them apparently, each apparently running at 2.00GHz. I have a sweet extra-large high-quality monitor. Default resolution is 1440 x 900 or something (though I usually run my games on default lower resolutions. I wonder if that results in some of the hiccups and crashes.)

So, questions about replacing hardware in my current computer:

1: Any ideas as to what might be wrong with my laptop's battery? All I know is one day I got an error message for the battery and since then a red light's been constantly flashing when it's plugged in (it used to flash a different color, I presume when it was charging or something.) It's clear the battery won't recharge, at any rate, and the computer dies the moment it's unplugged.

2: How is this whole convoluted upgrading process done? Could I purchase the parts and attempt this myself, or is this something a trained professional at a Geek Squad at Best Buy should be attempting? If so, how long does it take for them to install new parts? I really don't have the opportunity to hand over my computer for much more than 24 hours, max.

3: If I were to replace my video card, would this mess up my ability to access old playthroughs or games I had already installed with the former video card? In other words, is there a problem with the computer's ability to recognize a transition to new hardware, and does it effect everything previously installed? Will I need to run additional programs or will the computer recognize the new technology automatically?

4: If I no longer have a copy of my Windows XP install disc, would there be problems with attempting to install a new hard drive? Or is there a way I could automatically convert all the files on my 'old' hard drive -- including an operational Windows XP, my installed games and their svaes, etc. -- to the 'new' hard drive? Alternatively, will I have to settle for merely transferring documents and save files to a blank CD, then copy-pasting everything to an empty hard drive? ME save files better be transferable, is all I'm sayin'.

5: Where should I purchase the parts I need? Amazon.com? Best Buy? How do I assure they're compatible with a Dell Inspiron 1520? Or is virtually every video card for a laptop compatible?

Thanks,
Snake