New Computer for old man

My topic title about says it all. I am a one game gamer. I have been waiting for SH4 ever since it was announced. I am 57 years old and do not know if I want to spend $1600.00 at Bestbuy buying a super gamer computer for just one game. I now have a P4 2.4gig, 1 and 1/2 G memory and a 256mb Ati X800 pro video card. I think to be able to appreciate the beauty of this game I need to upgrade the above mentioned items. Right now I play on medium graphics with no check marked boxes and no AA. My monitor will support the higher resolutions so that is not a problem. I thought updating just my memory and processor speed would do but not according to BB. My video card is AGP and will not fit in the Acer 1500; which contain 2 gig of 5300 DDR memory and a dual core Athlom amd 4400 processor. Therefore they suggested a geforce Nvidia 256mg card the 7800 to go with this Acer unit. By the time their geek squad and sales tax and 2 year warranty is added on I am up to $1400, The other $200.00 goes to 2 gig more memory totalling out to 4 gig total. I need the opinion of you fine folks here as that will determine if I am a crazy old fool or not. Any bad or good criticism would be useful. Thanks in advance Bw Chaney