Your parallel choices include:
1. SRAM(e.g. 628128)+Lithium Battery(e.g. CR2032)+Battery Switchover IC(e.g. MAX691)
Advantages:
- A typical SRAM(e.g./i.e. 120ns) is fast enough not to need wait states up to some 24MHz(std. 12 clocker)
You can find some nice info. on timing issues here.
Disadvantages:
- Need to replace the battery every few years Typically
- Batteries are said to contribute to EMI problems
- MAX691 and battery need extra PCB space and design complexity(sort of)
2. NVRAM(e.g. DS1245)
Advantages:
- Can be treated just like an immortal SRAM(as fast and as simple)
Disadvantages:
- So they are much more expensive
3. FLASH(e.g. AT29C010)
Advantages:
Disadvantages:
- Some old FLASH ICs need an extra 12v for the write operation
- Can't be written to on a byte-by-byte basis i.e. need to be written to with some 'bothersome' sector oriented code.
4. EEPROM(e.g. AT28C010)
Advantages:
- Can be treated like an SRAM
- Have a three-byte-algorithm accidental write protection feature
Disadvatages:
- Are not 'very fast'
5. FPRAM
I haven't eaten one so far, What does it look like?