Upgrading my Dell Laptop Inspiron 1720 video card - possible? Impossible? Suggestions?

i bought a Inspiron 1720 laptop a couple years ago and have been impressed with it so far but......

I just bought Grand Theft Auto 4 for PC but have got it home and don't think I can play it. The requirements are here:

http://en.wikipedia.org/wiki/Grand_Theft_Auto_IV#Windows_version

I know I have Windows Vista (Not sure if it is service pack 1 though - where do I find this?). I have Intel Duo Core 2 Core CPU 1.80Gh (think I am fine here but they recommend more) Is this easy (and cheap) to upgrade? Memory (not sure what this is refering to RAM?) I think this equals 2046MB (should be fine here). I have enough Hard Drive (requires 16GB) - got a pile of that. Think I am good with sound.

The only real problem is my video card. I have a NVIDIA GeForce 8400M GS. Dedicated video memory is only 128MB and I need 256MB at least.  Is it easy to change this out and get a better video card?  I have read a couple of forums online and guys have done it (on a laptop).  How much would I expect to pay (would rather a pro did it) and not me?  Should I do it?  What would you suggest?  I would like to get it for about 100-120 pounds all in.  Is this possible?  Or should I just return the game?

I really want to play this game and I am willing to put in around 100 pounds to do it.  Would like sime suggestions. Any help much appreciated.  I guess it would be processor and video card I would need to upgrade.