Generating various emission modes via DDS
DDS will generate just about anything. 

Max practical frequency depends mainly on your wallet size.

Processing power is obviously the issue.
 Amateurs can do 100's of MHz and mayhaps GHz.
 As you approach the clocking frequency of your system you become increasingly able to create only a single frequency square wave at clock frequency :-). ie you need enough "bits" to both populate a sine wave at multiple levels and to run the frequency  generation logic.  

I'd "mentate" that A3E and F3E (SSB, FM) would be "easy enough" using the basic technique but frequency would be lower for an IC of given gate speed, given the complexity of the waveforms needed. ie using the technology in the AD ICs below you are probably not going to get much beyond 100 MHZ. You can do the arithmetic to look at what sort of bit manipulation is needed to form signals of the type required. (It's all just sine waves in the end :-) ).

Using "Super Nyquist techniques" (clove of garlic worn near heart, bury at cross roads at midnight) the ICs below can be pushed to an indefinite multiple of their primary frequency range. See eg AD AN-939: 
Super-Nyquist Operation of the AD9912 Yields a High RF Output

Commercial 40 GHz DDS - really DDS + upconversion



The Analog Devices AD9910  clocks at 1 GHZ and can produce output to 400 MHz. About $45/1000

AD say:


Cheaper AD9956 notes when used with an external VCO, enables the synthesis of digitally programmable, frequency-agile analog output sinusoidal waveforms up to 2.7 GHz.
The AD9910 and AD9912 high speed DDS chips can both be clocked at 1GHz, allowing for output signals up to 400MHz in the baseband. The quality of the DDS output signal depends to a high degree on the quality of the DDS reference clock signal. Therefore a lot of attention has to go the 1GHz reference clock in order to make the DDS a success. The following criteria are important to the reference clock:


http://martein.home.xs4all.nl/pa3ake/hmode/img/dds_1ghz_narrow.jpg

From data sheet (too much to format prettily - feel free :-) )

1 GSPS internal clock speed (up to 400 MHz analog output) 
Integrated 1 GSPS, 14-bit DAC 
0.23 Hz or better frequency resolution 
Phase noise ≤ −125 dBc/Hz @ 1 kHz offset (400 MHz carrier) 
Excellent dynamic performance with  


  80 dB narrow-band SFDR
  Serial input/output (I/O) control 
  Automatic linear or arbitrary frequency, phase, and 
  amplitude sweep capability 
  8 frequency and phase offset profiles 
  Sin(x)/(x) correction (inverse sinc filter) 
  1.8 V and 3.3 V power supplies 
  Software and hardware controlled power-down 
  100-lead TQFP_EP package 
  Integrated 1024 word × 32-bit RAM 
  PLL REFCLK multiplier 
  Parallel datapath interface 
  Internal oscillator can be driven by a single crystal 
  Phase modulation capability 
  Amplitude modulation capability 
  Multichip synchronization


You'd think they didn't know how to lay out PCBs, if you didn't know better ... :-) !





Digikey's stable](http://search.digikey.com/us/en/cat/integrated-circuits-[ics/interface-direct-digital-synthesis-dds/2556393?k=dds)

Digikey's champion - 2.7 GHz  AD9956
 $34/1 in stock.

PRODUCT OVERVIEW 

The AD9956 is Analog Devices’ newest AgileRF synthesizer.
The device is comprised of DDS and PLL circuitry.

The DDS 
features a 14-bit DAC operating at up to 400 MSPS and a 48-bit 
frequency tuning word (FTW). The PLL circuitry includes a 
phase frequency detector with scaleable 200 MHz inputs
(divider inputs operate up to 655 MHz) and digital control over 
the charge pump current. 

The device also includes a 655 MHz 
CML-mode PECL-compliant driver with programmable slew 
rates.

The AD9956 uses advanced DDS technology, an internal 
high speed, high performance DAC, and an advanced phase
frequency detector/charge pump combination, which, 


when 
used with an external VCO, enables the synthesis of digitally 
programmable, frequency-agile analog output sinusoidal waveforms up to 2.7 GHz. 


The AD9956 is designed to provide fast 
frequency hopping and fine tuning resolution (48-bit frequency 
tuning word). Information is loaded into the AD9956 via a
serial I/O port that has a device write-speed of 25 Mb/s. The 
AD9956 DDS block also supports a user-defined linear sweep 
mode of operation.  

The AD9956 is specified to operate over the extended
automotive range of −40°C to +125°C.