Thanks everyone for the feedback. This has helped me to shape my design. I am reconsidering using BASIC exclusively in favor of using a monitor program that will allow me to assemble my own programs. I am familiar with assembly and, though more tedious, in the long run it will be a more powerful solution than BASIC only. That's not to say that I'm giving up on BASIC entirely. Neil put it well when he said referred to what I want as a “little pc.” I want a small battery powered computer that I can take places mainly for hobby purposes. I have written several programs over the years for a graphing calculator that I use (to manage finance and stocks, etc.), and I think it would be fun to build my own computer and port those programs there (I know, I have a strange definition of the word “fun”). Richard, you mentioned that a PS/2 keyboard will not make my computer very portable. I would prefer a small keypad (as I mentioned in my original post) and right now I'm really hoping for something that will be a portable but desk-top unit. A side goal is also to make this system inexpensive (less than $100), this is not for budget so much as the novelty of having a home-built inexpensive math machine.

I'm going to take this project in 3 phases. For the first phase, I've researched 8052 interfacing, and I'm playing with an emulator to get a good idea of how exactly my system is going to 'fit' together. For the second phase, I'll wirewrap a simple board with the microcontroller, EEPROM (4KB), SDRAM (64KB – 59KB mapped), logic, address bus latch and any additional support circuitry I'll need to for RS232. It will have a serial terminal interface, so I can start writing any software I need and make sure everything is working together before I interface my LCD and keyboard. I've purchased a 640x64 dot 2-font text LCD and an AT89C51 (I realize this rules out BASIC52), as well as an EEPROM programmer. The final phase will be to build the computer with all the peripherals and getting them to fit together and into a wooden case. After my computer is together and it works, I'll start a design for a 'removable' memory storage, whether it will be a removable EEPROM that I can store programs on, or a floppy drive (I've read the interfacing is pretty complicated, though), or perhaps compact flash. But that will be another project-- phase 4 of my 3-phase plan.

Here's the memory map of the system I intend to build:

Internal RAM
00-7F Registers, Bit variables, Stack, etc.

External RAM & Code memory
0000-0FFF (4KB) EEPROM
1000-FBFF (59KB) RAM, user variables, and programs will run from here (ultimately, programs will be loaded here from the external memory before being executed). I was thinking of using the first 2KB for vars and starting at 1800 use for program storage.
FC00-FFFF memory mapped devices (I will use this to control my external storage medium, whatever that may be)

At stage 3, I may consider moving the monitor or OS program from the 4KB EEPROM to the internal EEPROM, and recovering that space for RAM. After that point, my programs may use 0000-17FF (6KB) for variables (as apposed to the 2KB in phase 2). My understanding is that I can still address 0000-0FFF as XRAM even if I'm using external code memory, and using the processor's built-in 4KB for code memory.