If you have $500 to $800 to spend and time to really shop wisely buy a new laptop. Intel not long ago released the T8000 and T9000 series notebook line of CPUs to replace the T5000 and T7000 lines. The new lines do not contain any major breakthroughs over the reigning top of the line T7000 series CPUs which would more then meet your needs.
My son and I went to Best Buy for something the other day and in the past you might see one or two Sony notebooks in stock. This time an entire wall was devoted to SOny with the new CPUs. They have totally dropped the T7000 series almost overnight in what had to be a great deal with Intel. At the same time the used and refurbished snotebook markets swelled with Sonys equipped with T7000's at deep discounts.
I have seen incredible deals on eBay and eBay stores like PC Connection Online Auctions, BigCityDirect and others that buy refurbished laptops in bulk. If you pick up a refurbished laptop while it is still under the 90 day refurbish wqarrenty, you are also eligible to purchase a company extended warranty. Just take your time looking.
Finally, as examples, this week I purchased a refurbished Sony 17" with a T7700 CPU and the ATI Mobility Radeon HD2300 for graphics (a $2,500 to $3,000 computer 6 months ago, and which is still being sold for $1,800 buy online retailers, including outlet stores like Overtock.com) for $849 on eBay. I also purchased a HP COmpaq 6820s with a T7300 CPU, 17" screen, 2gb RAM, Vista business, all orginal packaging (including Vista disks and driver disks and which package was sealed and had never been opened, for $620.00). Once again this computer could not have been touched for the price a few weeks ago and the T7300 CPU is plenty fast and also will multi-taks with its two cores and meet your needs long into the future. If you have the money to spend, now is the time to buy a T7000 series notebook (the higher number in the 7000's the better) built by Sony, but I have made and seen great deals on HP Compaq, Acer and other brands, just do the legwork. Maintaining an old laptop can be expensive as often even the memory is no longer made or the BIOS limits you to 1gb and your hard drive is reaching a critical stage where you could loose important items built up over the years, even if you have a backup.
Hope this helped.
ock.com is still selling for $2,000) for $890,00. A month ago it was a top of the line laptop, and today does not differ very much from the new one, and has everything you would every need for multi-tasking, etc. For less, I purchased a HP COmpaq 6820s, T7250 CPU, discrete graphics, 17" screen and in perfect shape with all litature and orignal Vista disks for $620.00. Bottom line for the next week or so you should be able to pick up a computer worth in terms of performance and price a few short weeks ago a few thousand for between $600 and $850. Upgrading an old laptop just does not seem like the right move at this point in time given the market assuming you have some money to spend.