I have to agree with the earlier posters that taking one level of spell flux (at least) is necessary. In order to back up this opinion, I'll crunch some numbers for you.
Spell Flux:
Damage: 50/70/90/110/130 (50 damage + 20 per level)
Ap ratio: .33 (this is static for all spells, and doesn't improve with leveling)
Note: Bounces up to 6 times, and applys a -15 mr debuff for 5 seconds on each bounce. (can stack up to 5 times)
Overload:
Damage: 70/105/140/175/210 (70 damage +35 per level)
Ap ratio: .45 (this is static for all spells, and doesn't improve with leveling)
Note: Also adds 10% of your max mana as damage. (this is static, and doesn't improve with leveling)
Now, to prove that taking a level of spell flux is better than having one more level of Overload, allow me to list some examples.
You build with no cooldown reduction, so say you're only able to cast overload once, as a "burst damage" ryze. Okay, so we'll compare the effect of casting a lvl 2 overload, compared to casting a lvl 1 overload and a lvl 1 spell flux.
Lets assume you have...
100ap
1000 mana
Lvl 2 overload does 250 damage (105 + 45 from ap + 100 from mana)
Lvl 1 Overload does 215 damage (75 + 45 from ap + 100 from mana)
Now... level 1 spell flux does 83 damage... if you only hit once with it. (50 + 33 from ap)
If your goal is to drop an enemy as fast as possible, why would you cast one spell to do 250 damage, when you could cast two and do (at least) 298? This is not even mentioning the fact that Spell flux has waaaay more synergy with Ryze's ultimate than Overload does. Bouncing damage/debuff spell that now also does a big AoE on each bounce? Deadly.
The only downside to casting the two spells is that you'll burn through your mana faster, but if you're playing a burst damage Ryze, then you need to be able to throw out as much damage as possible as quickly as possible.
I would suggest switching your skill build to:
1. Overload
2. Rune prison
3. Spell Flux
4. Overload
5. Overload
6. Desperate Power (and then rank them up in the Desperate power > Overload > Rune Prison > Spell flux order)
If you find that you need your rune prison duration to hold a person long enough to drop them with burst damage, then instead maybe try:
1. Overload
2. Rune Prison
3. Rune Prison
4. Spell flux
5. Rune Prison
6. Desperate Power (and then Desperate Power > Rune Prison > Overload > Spell Flux)
For my personal build of Ryze, though, I always go with:
1. Spell flux
2. Rune Prison
3. Overload
4. Rune Prison
5. Rune Prison
6. Desperate Power and then Desperate Power > Rune Prison > Spell Flux > Overload
Most of Overload's damage comes from its ap gain and it's 10% max mana, more than it's actual +damage/level, so I typically find that Overload is a 1 point wonder for most of the game.