Depending on your memory and smartnets, etc. you can purchase the parts needed to upgrade the PIX to version 8 code. I believe the latest PIX code is 8.0.2. That being said, I got an e-mail yesterday saying there is a trade in program for the PIX to ASA.
I got a quote recently to take a pix 515e with v6.3.4 to the version 7/8 train. It was $1200 after the rom and flash mem upgrade, license upgrade, and smartnet renewal. The ASA 5510 is about $2300, and it is not being End of Lifed like the Pix. As such, we decided to get the ASA5510. The ASA model uses the ASDM which is a lot better than the traditional PDM.
You need 16 meg of flash and 64meg of ram. Plus you have to upgrade from 6.3.4 to 7.2 then to 8.02. Here is a link.
http://www.cisco.com/en/US/docs/security/pix/pix80/release/notes/pixrn80.pdf
Take a look at the memory and software requirements section.