Please help me to upgrade my AMD Athlon 3700+

Packard Bell iPower 6600SL.
More about this:

AMD Athlon 3700+ 2.2 Ghz Socket 939
Motherboard: NEC Computers International GA-K8N Ultra SLi
RAM: DDR PC3200 2 GB(1 GB into 2) 2 more slots free.
Graphics with dedicated memory: 2 into Nvidia Ge force 6600GT 128MB.
Windows XP Home edition SP3.
360GB HDD.

I am using this system for more than 3 yrs.

Please help me with an upgrade in components.

I am generally happy with this processor.
My main complaint is the noise of fans, because specification is for a gaming PC. But I don`t do any games now.
This motherboard is big. I need to to change cabinet.
What are the motherboards which I can use possibly with a DDR2 or better with AMD Athlon 3700+ 2.2 Ghz Socket 939?
What`s your suggestion on keeping my one Nvidia Ge force 6600GT 128MB graphics card as I am not doing much games? PLease suggest other graphics card with DVI output.
What is the importance of memory of graphics card? is that for Video..
What cabinet I should look for. I like a quieter one?
Thank you in advance

 

As for the case, I use an Antec P-180 with 4-120mm fans and a Volcano 9 HSF and with the fans set at mid-speed, except for the lower 120, which is set on high for the HDD, this is one of the quietest cases you can buy. Here's the link to the Antec P-182, which is identical to the P-180, just a different color. The P-180 is silver and the P-182 is gun metal black. http://www.newegg.com/Product/Product.aspx?Item=N82E16811129025 This case does not have a PSU suppiled. BTW, this case comes with 3-120mm, tri-speed fans already mounted. I just added another one in the front. Since the PSU is mounted on the bottom of the case, to cut down on noise and heat, you'll need a PSU with a bit longer MB power cord. I use the Antec 'Neo-Power 550.' It not only has longer MB power cords, but is also modular cabling which cuts down on all the cable clutter within the case.