Eking ~ 1 more year out of a primitive system

You'll need to post more specs, as few 4+ year old computers will be able to run NVN2 without some substantial upgrades.

No, you won't be totally bottlenecked by the AGP.

You will probably be bottlenecked by the CPU or the RAM - but the inclusion of a better gfx card will probably make things an awful lot faster.

As for running NWN2... you may not be in luck.

More specs needed!

To give you an impression of how bottlenecked that card would be by your computer..

My friend owned an P4 2,4ghz, with 1gb 266mhz ddr ram, and an 4x AGP port.

He had a newly bought ATi X1600Pro in that box, wich ran stable and considerably faster than his old system. However, when he upgraded to a
AMD x2 3800+ he got a considerable boost, but that pos ati card started overheating, it received a proper workload and got too hot  
From about 60c to over 85, with no other changes than cpu and proper ram.
So you'll get at best 50% out of that card, but your drives will still load too slow, and your cpu will process all the new textures too slowly etc..

I'm already in the process of upgrading the system ram, so it will be 1GB PC2700 DDR1 as of tomorrow. The processor is a trusty but suitably ancient AMD Athlon XP1800+ (1.5 Ghz).

The power supply is 300W. I'd probably have to upgrade that too?

Not entirely sure you'll need to upgrade the PSU, the main benefit, - assuming the one you have, - can handle the strain, is that the exhaust air will be cooler out of a stronger PSU.

You will definetly not have to upgrade the PSU. I have a 3.7 Northwood with 1gb RAM, a 6800gt (a very power hungry card), a CD drive and 2 H/Ds running off a 350 Watt emachines PSU.

You're going to be hurting I would suggest just waiting until the game is out then see the reviews on how the game performs with older PC. After that you can decide whether it worth it or not to upgrade. However it your hell bend I would buy the cheapest "good" card you can find some where along the lines of a x800GTO card, maybe a 6600GT or if you can find a 7600GS for pretty cheap. Your going to have to spend between 100-150 (I wouldn't pay more and assuming your in the us) for a card I think the GS are going for about 140 with out rebate and the GTO's for about 120-130, you may have to upgrade your PSU (not sure depends on card you purchase however most like a 350PSU).

Okay, slightly different question: if I was going to upgrade the CPU without getting a new Motherboard, the most advanced thing I could get, I think, would be an Athlon XP 3200+ (2.2 Ghz), the last generation CPU to use the socket A format. Intuitively, I would expect a relatively modest gain in performance over the XP 1800+ (1.5 Ghz). Am I wrong?

Possible new components so far:

1 GB RAM
New video card
New CPU
New Motherboard

This is starting to look like a "time to get a new PC" situation ...

you should save up and buy a new rig at christmas, i dont see much point in delaying the inevitable.