Can I update GPU but not RAM/CPU/Mobo?

I very much doubt that you could run HD properly on your rig due to the P4,(guess it depends on how well it overclocks). Your best chance would be to find an ATI card, im guessing its going to need to be AGP so probably a 3650 would be your best shot, last i heard they(ATI cards) were best at offloading the CPU load, im also guessing your PSU isnt up to much if it came with a system that is 1999 vintage so find one that dosent need extra power which will probably be a GDDR2 version but if you can find a GDDR3 card that dosent need power then get that for preferance.
You really are on the minimum requirements for that kind of thing (HD movies) but games wise it should be ok.

Mactronix

Um yeah with your current setup upgrading your video card would be almost pointless since youll have many bottle necks. outlw6669 is your best advice unless you want to buy a highly upgradable mobo and fit it with some lower quality componants with alot of ehadroom for the future. but if you upgrade only once every 10 years even that wouldnt make much sense. just do what he said and buy a low-mid rang computer. At least if there is a bottle neck there you can upgrade it for vary little relativly.

Do yourself favor and don't spend a dime on an AGP card. It's time to upgrade and keep the old one as a backup machine.

Here is a solid system, that anyone can be proud of, that I just built on the cheap, or what I consider the cheap. It will play any game out there, not necessarily on high, but if you want more you can buy a better graphics card.

Antec 300 Case $59.99
Antec 761345-75093-6 92mm Case Fan $9.99
GA-EP45-DS3L $99.99
Q6600 G0 $189.99
G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 $66.99
Western Digital Caviar SE16 WD6400AAKS $79.99
SAMSUNG Black 22X DVD Burner $27.99
XIGMATEK HDT-S1283$36.99
XIGMATEK ACK-I7751 Retention Bracket $6.99
PC Power & Cooling S61EPS 610W $89.99 after $20.00 MIR

Get the video card of your choice. I recommend an HD4850.

Total price, net of video card and tax/shipping $670

An HD4850 video card would be at or about $200.00, more or less. You can go cheaper and upgrade later if you want.

Total net of Tax and shipping $870.00.

Why spend $150-$200 on an outdated card that will be bottlenecked on your POS system anyway. Put that money into a new solid quad core system that will carry you another nine (NINE? Did I just say that?) years.

The one I just built gave the the same "FREE" OC to 3.0G as they all will. I just locked the PCIe to 100Mhz and set the RAM to 2.4 (2 if you wish) and raised the FSB to 333. No voltage changes needed.

Don't forget to turn off CIA2.

Yeah, if you are on a tight budget then outlaw6669's will do, but it is no match for the system I posted, not even close. That is assuming you have the cash.