Plain enhancement bonuses on weapons generally affect only attack and damage, nothing else (barring a specific class feature, feat, or whatever). The Keen weapon enhancement is the way to improve the threat range. The Improved Critical feat or the keen edge spell can be used to replace the Keen enhancement.
In 3.0, critical threat extenders often did stack, so you could get ranges quite a lot larger, and you were likely to critical on any roll that actually hit.
In 3.5, none of these stack with one another, so generally speaking 15-20 is the best range you can get (e.g. +1 keen rapier). There are a few scattered exceptions that stack, but they’re rare.1
As a result, crit-fishing is not usually seen as a particularly powerful archetype in 3.5. For the most part, even with a lot of investment, crits remain too rare to justify the investment unless you do find some of those exceptions. Even then, it usually only gets really dangerous when you do silly things like +1 Aptitude2 Kaorti-Resin3 Rapiers4 with Lightning Maces5 and Roundabout Kick.6
1 At least one, the Disciple of Dispater, relies on the fact that it was written for 3.0 and never updated for 3.5, which technically makes it legal as-is in 3.5 even though it has 3.0-style critical threat increases.
2 Aptitude is a weapon enhancement that allows the weapon to be treated as a different sort of weapon for feats.
3 Kaorti resin is a special material that can be used to make weapons. Weapons made from it are always Exotic, and always have a ×4 critical multiplier if they deal piercing or slashing damage. Frequently banned.
4 Rapiers are the traditional 18-20 threat range weapons, though there are probably better choices here.
5 Lightning Maces is a feat that gives you a free attack if you roll a critical with a mace. Since maces generally have a small threat range, this wouldn’t be so bad, but (arguably) Aptitude allows you to apply it to wider-range weapons. Even that’s not too awful, since it’s still not infinite or anything...
6 Roundabout Kick normally gives you a free Unarmed Strike attack after a critical. But with Aptitude, that can be another attack with your large-range, high-multiplier weapon. That both Lightning Maces and Roundabout Kick applies to. As a result, you get two free attacks for every critical. Mathematically, your odds of ever stopping go down dramatically after you succeed on the first few criticals, leading to nigh-infinite attacks. Now a build that was only so-so is literally breaking the game – not just by being powerful, but by literally creating a situation that the rules cannot handle.
Lightning Maces was famously banned in the Test of Spite after one of the most impressive matches: OloDemonsbane failed a save against death urge. He then proceeded to crit himself... and crit himself, and crit himself, and crit himself, until he eventually ran out of bullets, scoring nearly 45,000 damage on himself, all thanks to Lightning Maces. (technically he would have died before all the attacks completed but the number is more impressive if he uses all his attacks)