That old P4 does not owe you anything. She is ready for menial tasks like web browsing only.
Anyway, both computers are nice. Both will lighting fast compared to what you have currently. A couple of things. 64 bit can us up to 8 gigs of RAM. I see the Dell is offering only 2 gigs of RAM. I would go no less than 3 but recommend 4 gigs RAM. If you take a look at the Dell's case note the open cooling vent. Heat kills computers. Dell apparently understands this judging by the case were Gateway was more interested in putting bright orange color strips on the case and not a cooling vent. Also, the Dell looks much cooler
BTW, the link above for Cyberpower. The price I got by customizing the i7 included the EVGA 9800 GT 512mb vid card and 4 gigs of RAM with all the other goodies at the cost noted above. Here with the Dell, 2 gig of RAM and a 8800GT vid card. Price, $1200.00+. So you see, bang for the buck.
I think if you swing by Best Buy and really look over the Gateway you will see that it is just not that great. At least to me it was not great. Just felt like it would not last. If that is the case, then you have eliminated a contender.