Graphics, That card will bottleneck the whole system.
For that price though it’s a nice system, a better graphics card and PSU would be good though.
That’s a very fast CPU, what speed is the RAM do you know?
Also Maxtor Drives are known for being a bit dodgy, their bearings wear out quicker and they run very hot, try something from Seagate or Western Digital.
Also if you’re going with a 4000+ maybe look at a PCI-e Motherboard, would suit that system better!
If you don’t want to spend much on graphics then look for something like a 6600GT or even a cheap X800-Series Card.
Is this system pre-built or are you building yourself?
Thermaltake 425 watt- 525 watt psu
or Antec True power series 450 watt and above
these are two trusted brands known not to blow out your hard worked for system
also if you really dont play games or not willing to spend alot on a video card try a Geforce FX 6200 or X300 those are pretty decent cards
Western digital hardrive would be good and probably cheaper and be sure to pic up a couple of 80 mm fans for cooling