I want to upgrade my computer but I'm terrified that I'm going to bottleneck it somehow, so I hope someone who knows more about hardware can help me out

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Current specs:

Video Card: NVIDIA GeForce 7800 GTX 256mb
Processor: Intel dual core 3.0 Ghz. (not sure what model, is that significant?)
RAM: 1 Gb (Crucial 2x 512 mb DDR2)
Motherboard: Intel® Desktop Board D945GTP (http://www.intel.com/products/mother...5gtp/index.htm)
OS: Windows XP Media Center 2002 SP2 according to right clicking my computer (Windows XP Pro V 5.1.2600 according to help and support center computer information)

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I'm thinking to upgrade my RAM to 4 gigs and my video card to a NVIDIA 9800GT 512mb. Would this work well? Would I bottleneck anywhere? Is there anything I need to keep in mind when choosing RAM? Thanks.

P.S. I'm most concerned about the motherboard bottlenecking me. Don't know if that's a realistic fear or not.